window-port updates

Remove start_screen() and end_screen() from the
Window-port interface.

They were only ever used by tty, and there was a comment
carried to several window-ports about how they "really
should go away. They are tty-specific"

term_start_screen() and term_end_screen() are part of
terminal/NO_TERMS supporting routines now.

src/windows.c

@@ -598,8 +598,6 @@ static struct window_procs hup_procs = {
 #endif
     hup_get_color_string,
 #endif /* CHANGE_COLOR */
-    hup_void_ndecl,                                   /* start_screen */
-    hup_void_ndecl,                                   /* end_screen */
     hup_outrip, genl_preference_update, genl_getmsghistory,
     genl_putmsghistory,
     hup_void_ndecl,                                   /* status_init */