Call: don't listen to unreadMessageCountChanged

There doesn't seem to be a need for the Call to be updated (and
thus emit a changed signal) when the unread message count changes
in the peer ContatMethod. Otherwise all calls (current and history)
will keep getting updated with this signal.

Change-Id: I2f464ef39fe4e48f2b52c441a3f18ccb4b9a419f
Tuleap: #1076