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/eval.c

@@ -7469,7 +7469,6 @@ void ex_execute(exarg_T *eap)
     if (eap->cmdidx == CMD_echomsg) {
       msg_ext_set_kind("echomsg");
       msg_attr(ga.ga_data, echo_attr);
-      ui_flush();
     } else if (eap->cmdidx == CMD_echoerr) {
       // We don't want to abort following commands, restore did_emsg.
       int save_did_emsg = did_emsg;