Clean up remaining feature flags.

Problem to solve

We have a number of feature flags that need to be removed. See https://app.periscopedata.com/app/gitlab/792066/Engineering-::-Feature-Flags

Proposal

1. Cleanup the following:
   1. #349944 (closed)
   2. #346907 (closed)
   3. #346908 (closed)
2. Convert the following to ops as they need to stay around indefinitely with 0.01% rollout:
   1. detect_cross_database_modification
   2. query_analyzer_gitlab_schema_metrics