[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.

#!/bin/sh -e
if [ "$1" = "purge" ] ; then
        update-rc.d cubicweb remove >/dev/null
fi
 
#DEBHELPER#
 
exit 0