[API] removed sendLeaveMessage from ITransmitter IFACE

Put the logic into the SarosSession. This also removes the need for a
hack when sending leave messages as the host will perform the user list
synchronization.

Change-Id: I7f352af081c8ebd544dfb8385824868186139678
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1042
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>