refactor(ui): remove some superfluous ui_flush() calls

- <expr> mapping has no business saving and restoring the
  low-level UI cursor. The cursor will be put in a reasonable
  position after input is processed, chill out.
- TUI handles output needed for suspend
- vgetc() family of function does flushing

src/nvim/ui.c

@@ -472,11 +472,6 @@ int ui_current_col(void)
   return cursor_col;
 }
 
-handle_T ui_cursor_grid(void)
-{
-  return cursor_grid_handle;
-}
-
 void ui_flush(void)
 {
   assert(!ui_client_channel_id);