call_leg: remove unused event MSC_EV_CALL_LEG_RTP_RELEASED

The event is actually never dispatched and useless, because when an RTP stream
releases, the call_leg terminates directly anyway (which wasn't apparent when
starting to design the call_leg FSM yet).

Change-Id: I6b2fc1225c960fa2f7c46adf241520217a07821c
6 files changed