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 51d3082f

 "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>