[dataimport] massive_store: do not drop constraints with `cascade` since it breaks things We want to be able to restore constraints at the end of the import. If they are silently dropped due to a CASCADE, we can't do that.

usr/lib/python2*/*-packages/cubicweb/entities/
usr/lib/python2*/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/python2*/*-packages/cubicweb/*.py