feat(ui): use msg_grid based implementation for cmdheight=0

src/nvim/ui.c

@@ -614,12 +614,6 @@ bool ui_has(UIExtension ext)
   return ui_ext[ext];
 }
 
-/// Returns true if the UI has messages area.
-bool ui_has_messages(void)
-{
-  return p_ch > 0 || ui_has(kUIMessages);
-}
-
 Array ui_array(void)
 {
   Array all_uis = ARRAY_DICT_INIT;