[INTERNAL] Removed SessionIDObserv. in MUCNegoiationManager

The id is now obtained from the current Saros session object instead of
using this observable.

Change-Id: I84167830a07e182da897896726ec379b41e1c4b2
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1681
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>