BListView: do not spawn a thread on every mouse click.

That's a bad idea. The drag and drop tracking could get confused enough
to crash app_server (possibly because of calling InitiateDrag multiple
times?)

Fixes #14983.

headers/os/interface/ListView.h

@@ -189,9 +189,6 @@ private:
 			bool				_ReplaceItem(int32 index, BListItem* item);
 			void				_RescanSelection(int32 from, int32 to);
 
-			void				_DoneTracking(BPoint where);
-			void				_Track(BPoint where, uint32);
-
 private:
 			BList				fList;
 			list_view_type		fListType;