[INTERNAL] removed session reference from DirtyStateListener

The logic already keeps track when to install such a listener. There is
no need to check it again.

Change-Id: Id14f8f5209f88a97ec6eab9c42b40003fd19de1c
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1849
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>