Fix recent regression from 77e7bbc where an attachment on a received group notice with XmlRpcGroups messaging did not appear in the user's inventory.

This was because the "session ID" when the message template was copied was always replaced with the group ID, whereas a notice requires this to be the notice ID.
Instead just copy the "session ID" as is - other callers already have this set properly so replacing with group ID was redundant anyway.
Relates to http://opensimulator.org/mantis/view.php?id=7037