LibGUI: Remove UndoStack's automatic command combo'ing

UndoStack will now merge adjacent commands *if they want to be merged*
instead of bundling everything you push onto it until you tell it
to "finalize the combo."

This uses less memory and gives applications full control over how
their undo stacks end up. :^)

Userland/Applications/PixelPaint/ImageEditor.cpp

@@ -48,7 +48,6 @@ void ImageEditor::did_complete_action()
 {
     if (!m_image)
         return;
-    m_undo_stack->finalize_current_combo();
     m_undo_stack->push(make<ImageUndoCommand>(*m_image));
 }