[FEATURE] #91 Remove participant from session 

The feature was already in the code base but was not activated as
it does not work properly. Activated that feature now because the
necessary corrections were committed with commits
7b91fdd26adc00879ee90d6c7751bc8ac69af244 and
fc099fe3d80143457c7b0bc0469348de9c29e21f.
 
Change-Id: I793dbae31385dbb20e6176dc6ef35020045cc800
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1044
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>