Revert "Merge branch 'sc1-rate-limiting-migrator' into 'master'"

What does this MR do and why?

This reverts !106123 (merged) because the feature flag lookup became much more expensive:

    def feature_flags_available?
      # When the DBMS is not available, an exception (e.g. PG::ConnectionBad) is raised
      active_db_connection = begin
        ActiveRecord::Base.connection.active? # rubocop:disable Database/MultipleDatabases
      rescue StandardError
        false
      end

      active_db_connection && Feature::FlipperFeature.table_exists?
    rescue ActiveRecord::NoDatabaseError
      false
    end

The call in ActiveRecord::Base.connection.active? causes a connection to be checked out with a ; SQL query. When this happens in a number of endpoints, we saturate the connection pool and drive up the virtual transaction rate in PostgreSQL.

Relates to gitlab-com/gl-infra/production#8311 (closed)

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