MAINTAINERS: remove rt2x00.serialmonkey.com list and web page

rt2x00.serialmonkey.com will be shutdown.

Since traffic on rt2x00 mailing list is very low, we can use only
linux-wireless list for any rt2x00 related topics.

Thanks for Luis Correia, Ivo van Doorn and Mark Wallis for maintaining
rt2x00 servers for years!

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>