Staging: remove at76_usb wireless driver.

There is already an in-kernel driver for this hardware (since 2.6.30),
at76c50x-usb, and it supports all of the same devices.  So this driver
can now be deleted.

Acked-by: Kalle Valo <kalle.valo@iki.fi>
Cc: linux-wireless <linux-wireless@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>