[INTERNAL] removed IActivityProvider usage from ISarosSession

The session interface now only accepts IActivityProducer and
IActivityConsumer registration.

Change-Id: Ifa7d3a279a9e604014e781e90b01c8621fd14737
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1511
Tested-by: Jenkins CI
Reviewed-by: Bernd Bieber <brotlee@zedat.fu-berlin.de>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>