[INTERNAL] removed getSession method from User class

There is only one session active at a time. Furthermore
returning the session has no value as the User object
might be cached but already got orphaned (session object)
no longer valid.

Change-Id: I9baeaa8f0638d0293c9b93a6efe4acc8d2066692
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/912
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>