[migration] don't crash when deleting a non-existing rtype
When asked to delete a relation type, instead of checking if it was
computed or not, just delete both the CWRType and CWComputedRType.
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/