MAINTAINERS: wifi: rt2x00: drop Helmut Schaa

Helmut hasn't been responding to rt2x00 related emails since 2016,
remove him from rt2x00 mainterner list.

Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231019202651.518136-1-stf_xl@wp.pl