LLAgentWearables::userUpdateAttachments() is unecessarily expensive server-side

While debugging our server-side object deletion code I noticed that my attachment was getting deleted on login (right after construction) and then recreated again. I traced it down to an ObjectDetach message being sent right after login. Looking at the viewer code I found it was being sent by LLAgentWearables::userUpdateAttachments() which will make two identical lists of objects_to_remove and items_to_add, and will then send for the objects to be removed, followed by the same items to be re-added. If this is a mechanism to convince the simulator to send some missing data then it is an unecessarily expensive way to do so and we should figure out a better way.

This could explain some simulator lag spikes from logins. This misbehavior should be removed from the viewer or I'll be tempted to add server-side protection against it on login.