SL-19686 reduce delay when switching between folders

indra/llui/llfolderviewitem.cpp

@@ -1769,6 +1769,11 @@ BOOL LLFolderViewFolder::isRemovable()
 	return TRUE;
 }
 
+void LLFolderViewFolder::destroyRoot()
+{
+    delete this;
+}
+
 // this is an internal method used for adding items to folders. 
 void LLFolderViewFolder::addItem(LLFolderViewItem* item)
 {

indra/llui/llfolderviewitem.h

@@ -394,6 +394,7 @@ public:
 
 	// destroys this folder, and all children
 	virtual void destroyView();
+    void destroyRoot();
 
     // whether known children are fully loaded (arrange sets to true)
     virtual bool isFolderComplete() { return mIsFolderComplete; }

indra/newview/llinventorypanel.cpp

@@ -2195,8 +2195,8 @@ void LLInventorySingleFolderPanel::updateSingleFolderRoot()
         LLUUID root_id = mFolderID;
         if (mFolderRoot.get())
         {
-            removeItemID(getRootFolderID());
-            mFolderRoot.get()->destroyView();
+            mItemMap.clear();
+            mFolderRoot.get()->destroyRoot();
         }
 
         mCommitCallbackRegistrar.pushScope();