Remove FLAG_SYNCAGENT.

This was the easiest flag to remove: nothing ever checks it at all!

It was part of an abandoned early attempt to make Pageant requests
asynchronous. The flag was added in commit 135abf244 (April 2003); the
code that used it was #ifdef-ed out in commit 98d735fde (January 2004),
and removed completely in commit f864265e3 (January 2017).

We now have an actually working system for async agent requests on
Windows, via the new named-pipe IPC. And we also have a perfectly good
way to force a particular agent request to work synchronously: just
pass NULL as the callback function pointer. All of that works just
fine, without ever using this flag. So begone!

psftp.c

@@ -2769,11 +2769,7 @@ int psftp_main(int argc, char *argv[])
     bool sanitise_stderr = true;
     char *batchfile = NULL;
 
-    flags = 0
-#ifdef FLAG_SYNCAGENT
-        | FLAG_SYNCAGENT
-#endif
-        ;
+    flags = 0;
     cmdline_tooltype = TOOLTYPE_FILETRANSFER;
     sk_init();