[schema] fix composite deletion handling
Do not delete component entities if the composite is not deleted in
the same transaction.
Deletion semantics are thus: if the composite is deleted, the
components are deleted.
Closes #3463112.
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/