chore(ci): remove add-pr-comment step

[ronething]

Type of change:

• Bugfix
• New feature provided
• Improve performance
• Backport patches
• Documentation
• Refactor
• Chore
• CI/CD or Tests

What this PR does / why we need it:

Due to the fact that branch commits in the fork repository can not be able to perform PR comments due to permission issues, this step has been temporarily removed. If needed, you can directly view the corresponding CI's run results.

ref: https://github.com/apache/apisix-ingress-controller/actions/runs/16110659454/job/45453356946?pr=2461

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[Copilot]

Pull Request Overview

This PR removes the add-pr-comment submodule and the related CI step for reporting conformance test results, addressing permission issues with forked branches.

• Removed the add-pr-comment submodule reference from .gitmodules
• Deleted the CI step in apisix-conformance-test.yml that posted results as a PR comment
• Cleaned up the now-obsolete action folder under .github/actions/add-pr-comment

Comments suppressed due to low confidence (1)