powerpc/xmon: Remove unused xmon_expect() & xmon_read_poll()

It looks like xmon_expect() was used for doing xmon over a modem (!?),
that code was dropped in 2005 in commit 51d3082 "Unify udbg (#2)".

Once xmon_expect() is gone xmon_read_poll() is unused, drop it too.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/xmon/start.c

@@ -25,10 +25,3 @@ int xmon_readchar(void)
 		return udbg_getc();
 	return -1;
 }
-
-int xmon_read_poll(void)
-{
-	if (udbg_getc_poll)
-		return udbg_getc_poll();
-	return -1;
-}