Strip leftover attr.ib() sentinels when these= is passed

[gyanu2507]

Summary

If these= is passed, class-body attr.ib()s are not collected as fields. They were also not deleted, so C.b could be a private _CountingAttr instead of missing.

I still do not collect those as fields. I only strip leftover sentinels so they cannot leak.

Fixes #621

Pull Request Checklist

• I acknowledge this project's AI policy.
• This pull request is not from my main branch.
  • Consider granting push permissions to the PR branch, so maintainers can fix minor issues themselves without pestering you.
• There's tests for all new and changed code.
• Changes or additions to public APIs are reflected in our type stubs (files ending in .pyi).
  • ...and used in the stub test file typing_tests/baseline.py or, if necessary, typing_tests/mypy.py.
  • If they've been added to attr/__init__.pyi, they've also been re-imported in attrs/__init__.pyi.
• The documentation has been updated.
  • New functions/classes have to be added to docs/api.rst by hand.
  • Changes to the signatures of @attr.s() and @attrs.define() have to be added by hand too.
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
  • Documentation in .rst and .md files is written using semantic newlines.
• Changes have news fragments in changelog.d.