[API] removed deprecated method sendActivity from ISarosSession IFACE

This method was not used anywhere and so this is the best time to
remove it from the interface.

Change-Id: I3a1403c8679c2f1498bc199e33d2b0e2744ac53e
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1040
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>