[schema] Let RQLConstraint inherit from BaseRQLConstraint instead of RQLVocabularyConstraint
There is apparently no reason for RQLConstraint to inherit from
RQLVocabularyConstraint as it gets its `repo_check` method from
`RepoEnforcedRQLConstraintMixIn` and RQLVocabularyConstraint (which inherits
from BaseRQLConstraint) actually only overrides that method. So simplify
inheritance by removing a useless level of indirection.
Adjust cw_unrelated_rql accordingly, where it was that RQLConstraint inherited
from RQLVocabularyConstraint.