[CLI-3329] Removed unused files and code

[sgagniere]

Release Notes

Breaking Changes

• PLACEHOLDER

New Features

• PLACEHOLDER

Bug Fixes

• PLACEHOLDER

Checklist

• I have successfully built and used a custom CLI binary, with no linting issues identified from this PR.
• I have clearly specified in the Release Notes section above whether this PR applies to Confluent Cloud, Confluent Platform, or both.
• I have verified this PR with Confluent Cloud resources in a production environment, if applicable.
• I have verified this PR with Confluent Platform resources in an on-premises environment, if applicable.
• I have attached manual CLI verification results or screenshots in the Test & Review section below.
• I have added appropriate CLI integration tests for any new or updated commands and functionality.
• I confirm that this PR introduces no breaking changes or backward compatibility issues.
• Leave this box unchecked if features introduced in this PR are not yet available in production.

What

Remove unused files related to the release process. This includes everything under mk-files/ and scripts/, some Dockerfiles under docker/, some APT/RPM repo files, and lines in Makefile.

Opening as a draft until the corresponding pipeline work is completed.

References

https://confluentinc.atlassian.net/browse/CLI-3329

Test & Review

Since this isn't a code change, manual verification consisted of:

• building locally
• running the linter
• running unit/integration tests

This is to check that nothing necessary was removed.

[confluent-cla-assistant]

🎉 All Contributor License Agreements have been signed. Ready to merge.
_{Please push an empty commit if you would like to re-run the checks to verify CLA status for all contributors.}

[sgagniere]

I didn't delete these spaces, because doing so generates a messier diff for standard_build_layout.patch.