cubicweb #1973680 multi-sources can lead to deletion of local entities [resolved]

seen on jpl: bad planning of a query subsequent to deletion of an entity led to deletion of all local version...

pretty annoying

priorityimportant
typebug
done in3.13.7
load0.500
load left0.000
closed by#42a0b7398d31 [repository] when deleting relations of a deleted external entity, we should skip non-crossable relation (closes #1973680)