query_cache_for_load_balancing is no longer used

Job #950159026 failed for fc8e4d4f:

These feature flags appears to be UNUSED
If they are really no longer needed REMOVE their .yml definition
If they are needed you need to ENSURE that their usage is covered with specs to continue.
- query_cache_for_load_balancing
Feature flag usage check failed.
==> 'bundle exec scripts/used-feature-flags' failed (1) in 0 seconds.

The usage of this feature flag seemed to be removed with !50959 (merged).