i8257: remove cpu_request_exit irq

This is unused.  cpu_exit now is almost exclusively an internal function
to the CPU execution loop.  In a few patches, we'll change the remaining
occurrences to qemu_cpu_kick, making it truly internal.

Reviewed-by: Richard henderson <rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
