[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/bin/cubicweb-ctl usr/bin/
+
−usr/lib/PY_VERSION/*-packages/cubicweb/cwctl.py
+
−../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl