rndis_wlan: remove nickname support

Supporting wireless extension nickname is pointless
and no other modern driver supports this, so remove
it.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>