CallModel: disconnect from removed Call's signals

Once a call is removed (is over) we no longer need to be connected
to its signals. Notabely, this prevents warning logs about the
Call object not being found in the model if it changes.

Change-Id: I38281f2adceec10c8b5b751fb6f984d3084bf458
Tuleap: #1076
(cherry picked from commit 6b787030)