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 <[e-mail]>
Signed-off-by: Kalle Valo <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

MAINTAINERS

···
 
 RALINK RT2X00 WIRELESS LAN DRIVER
 M:	Stanislaw Gruszka <[e-mail]>
-M:	Helmut Schaa <[e-mail]>
 L:	[e-mail]
 S:	Maintained
 F:	drivers/net/wireless/ralink/rt2x00/