DRTVWR-136: Remove redundant remove_(item|category)() functions.

Incoming changes from two different project repos left viewer-development with
LLInventoryModel::removeItem() and removeCategory() plus free functions
remove_item() and remove_category() in llinventoryfunctions.cpp.
remove_category() was actually the better implementation; migrated its body
into LLInventoryModel::removeCategory(). Clearly the previous state of affairs
-- with LLInventoryModel::removeItem() plus a remove_category() free function
in a very different source file -- fooled two different developers into
overlooking the other of the pair. Unfortunately we each added different
"missing" functions, leaving us with a complete set of four.
Fix existing references to remove_item() and remove_category() free functions.