Feature flags should be proactively removed to address technical debt.

See the feature flag guarding wiki for additional guidance.

Following a comment from aitor.esteve@mongodb.com below, we should remove the two following feature flags and all its usages, knowing that the value of this two flags is always true since 8.0 FCV: featureFlagTrackUnshardedCollectionsUponMoveCollection  and   featureFlag80CollectionCreationPath. Any code that is executed assuming that these features are not enabled is dead code and can be safetly removed.