Remove RealWaitForChar and mch_new_shellsize

The last occurrence of `RealWaitForChar` was replaced by the `os_microdelay`
function. `mch_new_shellsize` had an empty body, so there seems to be no reason
for keeping it around

src/ui.h

@@ -10,7 +10,6 @@ void ui_suspend(void);
 void suspend_shell(void);
 int ui_get_shellsize(void);
 void ui_set_shellsize(int mustset);
-void ui_new_shellsize(void);
 void ui_breakcheck(void);
 void clip_init(int can_use);
 void clip_update_selection(VimClipboard *clip);