Fix delete event error in postgres

An error happens when heat delete events in postgres,
 It seems that JOIN/DELETE is not supported in
"sqlalchemy/orm/".

Let heat delete events in postgres in the common
way.

Closes-Bug: #1301827
Change-Id: Id320abf15c4fd9f58e411ad7de7f044a9b5d1c1e
Merge "Fix delete event error in postgres"