MT#5473 Fix deleting peering groups.

Properly propagate deletion of hosts and their prefs, as triggers
don't hit in if they are deleted via constraints, so they leave
orphaned entries in the kamailio db.

(cherry picked from commit 8c3a27924771dc8b4d1136cff819611a3fe80e96)
MT#5473 bump changelog to 1.1.15 for backport to 3.1