session: fix duplicate session commit

* Removes duplicate code that has been moved to model.
  (PR #2264)

Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
session: fix commit after automatic table creation

* Fixes `commit()` call on database session object.  (PR #2265)

Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>