[INTERNAL] ensure a user is removed once

Ensure that components that try to remove users from the session
lately will not remove a user with the same JID that has rejoined the
session again.

Change-Id: I5ae7d67dd9f79f3cc2296b0473f6200f8fb352b1
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1219
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>