Support reindexing unique indexes

What does this MR do?

This removes the feature flag database_reindexing_pg12 to permanently enable the PG12 reindexing strategy based on REINDEX CONCURRENTLY. As such, this removes the old strategy and only cleans up code.

The feature flag and new strategy have been added with !64695 (merged).

Relates to #334372 (closed)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.