remove now obsolete comment

purge/matrixpurge.sh

@@ -96,8 +96,6 @@ purge_room_history () {
     sleep 5
   done
   printf "\nPurge finished with ${P_STATUSRET}\n"
-  # Remember that if you delete a room, it's state groups might still be around
-  # https://github.com/erikjohnston/synapse-find-unreferenced-state-groups
 }
 
 # do stuff for each room on server