Enable feature flag user_approval_rules_removal by default

What does this MR do and why?

This MR enables user_approval_rules_removal by default. It has been enabled on production since 2013-12-14: #432859 (comment 1694396850)

The background job ran successfully > 16 mil times and failed 3 times due to PG::QueryCanceled: ERROR: canceling statement due to statement timeout

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #434141 (closed)