[repository] when deleting relations of a deleted external entity, we should skip non-crossable relation (closes #1973680)
elses this may lead to things as described in the ticket...
#! /bin/sh -e#if [ "$1" = configure ]; then if which install-docs >/dev/null 2>&1; then install-docs -i /usr/share/doc-base/cubicweb-doc fifi#DEBHELPER#exit 0