wifi: remove orphaned rndis_wlan driver

Wireless RNDIS USB is a new-style CFG80211 driver for 802.11b and
802.11g USB hardware from around 2004 to 2006. This makes it more modern
than any of the others, but Kalle already classified it as "legacy"
in commit 298e50ad ("wifi: move raycs, wl3501 and rndis_wlan to
legacy directory").

Jussi Kivilinna worked on this driver between 2008 and 2012, and it has
only seen cosmetic updates after that.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@kernel.org>