[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...
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html