ice_transport: Nuke iceMutex_

iceMutex_ was used before because icest_ could change asynchronously.  This is
not the case anymore, since icest_ is initialized synchronously by
IceTransport::initIceInstance().

Furthermore, signaling of the iceCV_ condition variable is now done under the
group lock of the ICE transport, since it's under this lock that the state of
the session is changed.

Change-Id: I3f2b4b4141777b950c916553b45da880dd7056b6