Fix remove_messages deletes not cascaded.

Summary:
Deletes were not cascaded i.e. deleting a uid did not cause the corresponding message etc. to be deleted.
This was because of how the Query.delete() works in SQLAlchemy, see: http://docs.sqlalchemy.org/en/rel_0_9/orm/query.html#sqlalchemy.orm.query.

This commit fixes that by using session.delete() instead.

Test Plan: None.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D75