cli/flags: remove outdated TODO

[thaJeztah]

Libtrust was only used for pushing schema 2, v1 images, which is no longer supported; this TODO was likely left from when the CLI and daemon were in the same repository.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Merging #3915 (a89d4f1) into master (9906749) will decrease coverage by 0.37%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3915      +/-   ##
==========================================
- Coverage   59.22%   58.85%   -0.38%     
==========================================
  Files         287      285       -2     
  Lines       24726    24717       -9     
==========================================
- Hits        14643    14546      -97     
- Misses       9198     9287      +89     
+ Partials      885      884       -1     

[thaJeztah]

Oh! interesting; looks like we have a panic in a test (looks like it may need some fixes for t.Parallel()?)

#19 60.21 === Failed
#19 60.22 === FAIL: cli/command/container TestRemoveForce (unknown)
#19 60.22 panic: runtime error: invalid memory address or nil pointer dereference
#19 60.22 [signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xaf8060]
#19 60.22 
#19 60.22 goroutine 209 [running]:
#19 60.22 github.com/docker/cli/cli/command/container.TestRemoveForce.func1({0xc000470c60?, 0xc000479f78?}, {0xc88d86, 0xb}, {0xa0?, 0x9f?, 0x47?})
#19 60.22 	/go/src/github.com/docker/cli/cli/command/container/rm_test.go:25 +0x140
#19 60.22 github.com/docker/cli/cli/command/container.(*fakeClient).ContainerRemove(0xc88d86?, {0xdc98f8?, 0xc00012a000?}, {0xc88d86?, 0xc000479f78?}, {0x8?, 0x0?, 0x0?})
#19 60.22 	/go/src/github.com/docker/cli/cli/command/container/client_test.go:87 +0x4a
#19 60.22 github.com/docker/cli/cli/command/container.runRm.func1({0xdc98f8, 0xc00012a000}, {0xc88d86?, 0xc0001deec0?})
#19 60.22 	/go/src/github.com/docker/cli/cli/command/container/rm.go:65 +0x13e
#19 60.22 github.com/docker/cli/cli/command/container.parallelOperation.func2.1({0xc88d86, 0xb})
#19 60.22 	/go/src/github.com/docker/cli/cli/command/container/utils.go:156 +0x5f
#19 60.22 created by github.com/docker/cli/cli/command/container.parallelOperation.func2
#19 60.22 	/go/src/github.com/docker/cli/cli/command/container/utils.go:155 +0x1b5
#19 60.22 
#19 60.22 === FAIL: cli/command/container TestRemoveForce/without_force (unknown)
#19 60.22 panic: runtime error: invalid memory address or nil pointer dereference
#19 60.22 [signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xaf8060]
#19 60.22 
#19 60.22 goroutine 209 [running]:
#19 60.22 github.com/docker/cli/cli/command/container.TestRemoveForce.func1({0xc000470c60?, 0xc000479f78?}, {0xc88d86, 0xb}, {0xa0?, 0x9f?, 0x47?})
#19 60.22 	/go/src/github.com/docker/cli/cli/command/container/rm_test.go:25 +0x140
#19 60.22 github.com/docker/cli/cli/command/container.(*fakeClient).ContainerRemove(0xc88d86?, {0xdc98f8?, 0xc00012a000?}, {0xc88d86?, 0xc000479f78?}, {0x8?, 0x0?, 0x0?})
#19 60.22 	/go/src/github.com/docker/cli/cli/command/container/client_test.go:87 +0x4a
#19 60.22 github.com/docker/cli/cli/command/container.runRm.func1({0xdc98f8, 0xc00012a000}, {0xc88d86?, 0xc0001deec0?})
#19 60.22 	/go/src/github.com/docker/cli/cli/command/container/rm.go:65 +0x13e
#19 60.22 github.com/docker/cli/cli/command/container.parallelOperation.func2.1({0xc88d86, 0xb})
#19 60.22 	/go/src/github.com/docker/cli/cli/command/container/utils.go:156 +0x5f
#19 60.22 created by github.com/docker/cli/cli/command/container.parallelOperation.func2
#19 60.22 	/go/src/github.com/docker/cli/cli/command/container/utils.go:155 +0x1b5
#19 60.22 
#19 60.22 DONE 1565 tests, 2 skipped, 2 failures in 59.815s
#19 ERROR: process "/bin/sh -c gotestsum -- -coverprofile=/tmp/coverage.txt $(go list ./... | grep -vE '/vendor/|/e2e/')" did not complete successfully: exit code: 1
------
 > [test 2/2] RUN --mount=type=bind,target=.,rw     --mount=type=cache,target=/root/.cache     --mount=type=cache,target=/go/pkg/mod     gotestsum -- -coverprofile=/tmp/coverage.txt $(go list ./... | grep -vE '/vendor/|/e2e/'):
#19 60.22 github.com/docker/cli/cli/command/container.(*fakeClient).ContainerRemove(0xc88d86?, {0xdc98f8?, 0xc00012a000?}, {0xc88d86?, 0xc000479f78?}, {0x8?, 0x0?, 0x0?})
#19 60.22 	/go/src/github.com/docker/cli/cli/command/container/client_test.go:87 +0x4a
#19 60.22 github.com/docker/cli/cli/command/container.runRm.func1({0xdc98f8, 0xc00012a000}, {0xc88d86?, 0xc0001deec0?})
#19 60.22 	/go/src/github.com/docker/cli/cli/command/container/rm.go:65 +0x13e
#19 60.22 github.com/docker/cli/cli/command/container.parallelOperation.func2.1({0xc88d86, 0xb})
#19 60.22 	/go/src/github.com/docker/cli/cli/command/container/utils.go:156 +0x5f
#19 60.22 created by github.com/docker/cli/cli/command/container.parallelOperation.func2
#19 60.22 	/go/src/github.com/docker/cli/cli/command/container/utils.go:155 +0x1b5
#19 60.22 
#19 60.22 DONE 1565 tests, 2 skipped, 2 failures in 59.815s