Remove agent_schedule_callback().

This is another piece of the old 2003 attempt at async agent requests.
Nothing ever calls this function (in particular, the new working
version of async-agent doesn't need it). Remove it completely, and all
its special-window-message implementations too.

(If we _were_ still using this function, then it would surely be
possible to fold it into the more recently introduced general
toplevel-callback system, and get rid of all this single-use special
code. But we're not, so removing it completely is even easier.)

In particular, this system was the only reason why Windows Plink paid
any attention to its message queue. So now I can make it call plain
WaitForMultipleObjects instead of MsgWaitForMultipleObjects.

windows/winstuff.h

@@ -658,14 +658,6 @@ void handle_sink_init(handle_sink *sink, struct handle *h);
  */
 char *agent_named_pipe_name(void);
 
-/*
- * winpgntc.c needs to schedule callbacks for asynchronous agent
- * requests. This has to be done differently in GUI and console, so
- * there's an exported function used for the purpose.
- */
-void agent_schedule_callback(void (*callback)(void *, void *, int),
-                             void *callback_ctx, void *data, int len);
-
 /*
  * Exports from winser.c.
  */