Fix log_modified_related_models for nm-tables

Current entries in os_notify_log_t generated on DELETE of meeting_user/1 (user_id=1, meeting_id=1, group_id=2):

 operation |      fqid      |                updated_fields                | xact_id 
-----------+----------------+----------------------------------------------+---------
 update    | meeting_user/1 | {group_ids}                                  |    2647
 update    | group/2        | {meeting_user_ids}                           |    2647
 delete    | meeting_user/1 |                                              |    2647
 update    | user/2         | {meeting_ids,committee_ids,meeting_user_ids} |    2647
 update    | meeting/1      | {user_ids,meeting_user_ids}                  |    2647
 update    | committee/60   | {user_ids}                                   |    2647

Line 1 is obsolete and should not be generated if the instance gets deleted.

The line above was created by this notify trigger:

CREATE TRIGGER tr_log_nm_group_meeting_user_ids_meeting_user_t AFTER INSERT OR UPDATE OR DELETE ON nm_group_meeting_user_ids_meeting_user_t
FOR EACH ROW EXECUTE FUNCTION log_modified_related_models('group','group_id','meeting_user_ids','meeting_user','meeting_user_id','group_ids');

Start by locating the PL/pgSQL function log_modified_related_models and the tr_log_nm_group_meeting_user_ids_meeting_user_t trigger on nm_group_meeting_user_ids_meeting_user_t. Reproduce the DELETE of meeting_user/1 described in the issue and inspect how DELETE-related notifications are recorded. Done means the obsolete delete entry for the removed instance is no longer generated while the valid related-model updates remain.