session2: don't try to respond with reinvite when a stream dies

- terrible idea

darcs-hash:20090430083312-b2873-a06711d31ba3ca2abcbdb9e31b47b15c343f5cee
session2: don't include 'streams' in SIPSessionGotStreamUpdate because it always a reference to session.streams

darcs-hash:20090430083934-b2873-c148506f7798da2180c7e20557c4827a2693c95d