[INTERNAL] removed the session object from the user class

The session object has so many references that it may lead to memory
leaks if the user objects where cached somewhere and never gets GC.

Change-Id: I497361c6f3c193b35a6425036ec5dbdde4a4f848
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1070
Tested-by: Jenkins CI
Reviewed-by: Patrick Schlott <pschlottk@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>