[API] deprecated SessionIDObservable class

Because it was and is misused in all places the class is accessed.

Change-Id: I4963d66e248a6f5cb4144d0dbdcb797dc59120e1
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1140
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>