cubicweb-vcsfile #5706735 2.0 migration shouldn't be quite so aggressive [validation pending]

When dropping the VersionContent/DeletedVersionContent/VersionedFile entity types, we should make sure they're no longer in use by another cube.

prioritynormal
typebug
done in2.1.1
load0.500
load left0.000
closed by#1d13c5b5463e [migration/2.0.0] only drop etypes that aren't in fsschema
patch[migration/2.0.0] only drop etypes that aren't in fsschema [applied]