Clean up Credentials Inventory implementation after optimizations

What does this MR do and why?

optimize_credentials_inventory FF was enabled on GitLab.com. This MR removes this FF in favor of the implementation that is behind of this FF. As part of this, the MR cleans up implementation that was disabled by this FF, including credentials_inventory_pat_finder FF and its implementation(!193375 (merged), !194865 (merged)).

Database

In 18.5, which is a required stop, we added db/docs/batched_background_migrations/backfill_group_id_and_user_type_for_human_users_personal_access_tokens.yml and db/docs/batched_background_migrations/backfill_group_id_and_user_type_for_nonhuman_access_tokens.yml batched background migrations as part of optimize_credentials_inventory FF. This MR finalizes those BBMs.

References

• [Feature flag] Cleanup optimize_credentials_inv... (#575909)
• [FF] `optimize_credentials_inventory` -- Optimi... (#561260 - closed)
• [FF] credentials_inventory_pat_finder rollout (#545097)
  • Increase the performance of the PersonalAccessT... (!193375 - merged)
  • Make the /manage/personal_access_tokens GET end... (!194865 - merged)

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