Reduce complexity of issuable finder query.

What does this MR do?

This change removes a subquery from IssuableFinder that is not needed because it's redundant. This improves planning time (and execution time for some cases) for e.g. User#assigned_open_issues_count and User#assigned_open_merge_requests_count.

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/37125

Possibly also https://gitlab.com/gitlab-org/gitlab-ce/issues/37126