LP#1379824 Make PermaCrud.js disconnect() actually disconnect

The disconnect() method in PermaCrud.js was meant to wrap the
underlying session disconnect, but it never actually disconnected
the session.  This could lead to problems in long-living PermaCrud
objects, as they may think they are still connected when they are not
(the session remote_id is never cleared).

Also, remove a couple (now redundant) manual session.disconnect() calls.

Signed-off-by: Dan Wells <dbw2@calvin.edu>
Signed-off-by: Yamil Suarez <yamil@yamil.com>
Signed-off-by: Ben Shum <bshum@biblio.org>