Geo: Remove transaction lock in ProjectRepository

What does this MR do and why?

Removes the prevent_concurrent_inserts advisory lock callback from Geo::ProjectRepositoryState.

This is step 4 (Milestone 3) of the deduplication implementation plan for project_repository_states. The advisory lock was introduced in Milestone 1 to prevent duplicate records during concurrent inserts. Milestone 2 (released in 18.11, a required stop) deduplicated existing records and replaced the non-unique index with a unique index on project_repository_id.

Now that the unique database constraint is in place, the advisory lock is no longer needed and can be safely removed per the deduplication strategy documentation.

References

• Closes #590459 (closed)
• Deduplication migration: db/post_migrate/20260316153056_deduplicate_project_repository_states.rb
• Unique index migration: db/post_migrate/20260317141445_add_unique_index_to_project_repository_states.rb

Screenshots or screen recordings

Not applicable — no UI changes.

Before	After

How to set up and validate locally

1. Verify the unique index exists on project_repository_states.project_repository_id:

   ActiveRecord::Base.connection.indexes(:project_repository_states).map(&:name)
   # Should include 'unique_index_project_repository_states_on_project_repository_id'

2. Confirm Geo::ProjectRepositoryState no longer calls prevent_concurrent_inserts before validation.

MR acceptance checklist

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