Revert "LibGUI: Fix undo stack reporting wrong modified state"

This reverts commit 0b7e19e2bb.

Let's reverse the direction of the undo stack to fix the confusion.

Userland/Libraries/LibGUI/UndoStack.h

@@ -32,8 +32,6 @@ public:
     void clear();
 
 private:
-    size_t non_empty_stack_index() const;
-
     struct Combo {
         NonnullOwnPtrVector<Command> commands;
     };