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/popupmenu.c

@@ -1092,7 +1092,6 @@ void pum_show_popupmenu(vimmenu_T *menu)
     pum_is_drawn = true;
     pum_redraw();
     setcursor_mayforce(true);
-    ui_flush();
 
     int c = vgetc();