Fix Dockerfile.publish.test.centos7

[leezu]

Delete lines missed in prior commit & add CI test to prevent enforce no such mistakes are made in future

[mxnet-bot]

Hey @leezu , Thanks for submitting the PR
All tests are already queued to run once. If tests fail, you can trigger one or more tests again with the following commands:

• To trigger all jobs: @mxnet-bot run ci [all]
• To trigger specific jobs: @mxnet-bot run ci [job1, job2]

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CI supported jobs: [edge, centos-cpu, unix-gpu, sanity, windows-gpu, centos-gpu, windows-cpu, website, unix-cpu, clang, miscellaneous]

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Note:
Only following 3 categories can trigger CI :PR Author, MXNet Committer, Jenkins Admin.
All CI tests must pass before the PR can be merged.

[ChaiBapchya]

Thanks for adding that test so as to ensure docker-cache issues are caught earlier.
So basically now if someone wants to make any changes to docker-cache it can be verified using miscellaneous pipeline test.

[leezu]

Mainly the tests ensures that all dockerfiles are valid. Not only the ones used in some CI pipeline, but also the ones only used in CD