fix(autocmd): clean up autocmds only when needed (#17593)

src/nvim/autocmd.c

@@ -239,7 +239,6 @@ void aupat_del_for_event_and_group(event_T event, int group)
     }
   }
 
-  au_need_clean = true;
   au_cleanup();  // may really delete removed patterns/commands now
 }