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.

psftp.c

@@ -2430,12 +2430,6 @@ static bool verbose = false;
 
 void ldisc_echoedit_update(Ldisc *ldisc) { }
 
-void agent_schedule_callback(void (*callback)(void *, void *, int),
-                             void *callback_ctx, void *data, int len)
-{
-    unreachable("all PSFTP agent requests should be synchronous");
-}
-
 /*
  * Receive a block of data from the SSH link. Block until all data
  * is available.