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/option.c

@@ -1549,7 +1549,6 @@ int do_set(char *arg, int opt_flags)
     silent_mode = false;
     info_message = true;        // use os_msg(), not os_errmsg()
     msg_putchar('\n');
-    ui_flush();
     silent_mode = true;
     info_message = false;       // use os_msg(), not os_errmsg()
   }