Use model_query() instead of session.query in db.instance_destroy

Unify db.instance_destroy() method to use model_query() as others
methods use.

Side effect we won't try to soft delete already soft deleted instance

Change-Id: I36d5bb42b16901c260ff4276e31a6aec46ffa8a4
Merge "Use model_query() instead of session.query in db.instance_destroy"