Remove unused ProjectPushRuleFinder

What does this MR do and why?

Removes the unused ProjectPushRuleFinder class, its spec, and its entry in the finder collection allowlist.

ProjectPushRuleFinder was introduced to read from the project_push_rules table. Its only call sites were added in the read switch, which was later reverted in #592187 (closed) — but the revert left the finder class itself in place. Nothing has referenced it since.

This is the first of several small cleanup MRs for #605248 (closed).

Also removes the corresponding entries from .rubocop_todo/gitlab/bounded_contexts.yml and .rubocop_todo/gitlab/namespaced_class.yml.

References

• Issue: #605248 (closed)
• Parent tracking issue: #499156
• Read switch revert: #592187 (closed)

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