docs: fix contributing.rst reference to Session.remove()

(Second to) last example of Session.remove was removed in 1a409593f352.

docs/contributing.rst

@@ -236,8 +236,8 b' Each HTTP request runs inside an indepen'
 as in an independent database transaction). ``Session`` is the session manager
 and factory. ``Session()`` will create a new session on-demand or return the
 current session for the active thread. Many database operations are methods on
-such session instances - only ``Session.remove()`` should be called directly on
-the manager.
+such session instances. The session will generally be removed by
+TurboGears automatically.
 
 Database model objects
 (almost) always belong to a particular SQLAlchemy session, which means