Remove bazel from s2a-go repo

[xmenxk]

#99
#102

1. deleted all existing bazel setup: BUILD files, WORKSPACE file
2. removed bazel related kokoro tests
3. moved .proto files into own folder, and regenerated the .pb.go files
4. placed test key/cert pem files in the same package directory of the test.go file where they are embedded

Note: 3 and 4 are to make gazelle happy so that it generates proper BUILD.bazel files

• it doesn't like multiple .proto files in the same folder
• it can not find an embedded file if the file is outside of the package directory

Testing:

• go test:
  go build ./...
  go test ./...

• bazel test:
  // generate BUILD.bazel files with gazelle
  bazel run //:gazelle -- update-repos -prune -from_file=go.mod
  bazel run //:gazelle
  bazel build ...
  bazel test ...