wifi: atmel: remove wext style at76c50x drivers

Atmel at76c502/at76c504/at76c506 is a PIO-only (PCMCIA, mini-PCI
and Cardbus) 802.11b driver with incomplete CFG80211 support.

Both PCMCIA and WEXT are deprecated, and there is little chance
that anyone is still using this driver, so remove it.

The related at76c50x USB driver uses MAC80211 and remains.

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