Remove ActiveRecord QueryCache patch

The bug in http://github.com/rails/rails/issues/36587 is now fixed in https://github.com/rails/rails/pull/36618

The patch in lib/gitlab/patch/active_record_query_cache.rb and config/initializers/active_record_query_cache.rb will not be required once we upgrade to the next stable release (5.2.4 or 6.0.0)