[API] removed stop method from ISarosSession interface

This method was already marked for internal use and was luckily only
used by the SessionManager class which should be the only class that is
aware of this method.

Change-Id: I0e07b3a600c006049a7b0ae5977bc8927686cb8a
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1324
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>