manager: Eliminate unnecessary code, simplify sessions in stasis callbacks

manager: Eliminate unnecessary code, simplify sessions in stasis callbacks

Due to stasis filtering the stasis callback for AMI type messages is
guaranteed to only receive messages that can be turned into AMI events,
so remove the check done in the callback.

The sessions container usage for the stasis callbacks has also been
simplified by having a reference on the message router subscription
instead of having to acquire the sessions from the global object each
time.