linux-firmware: remove deprecated ar9170usb firmware

The ar9170usb driver is deprecated and was deleted from the kernel tree
(on Linux 3.0 (aka 2.6.40)). As a result, this firmware is no longer
used or required, at least as far as helium is concerned.

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=b0006e69615868f3dfdfe2bd64eb11973f1208fc

http://wireless.kernel.org/en/users/Drivers/ar9170

Bug #265 http://bugzilla.vyos.net/show_bug.cgi?id=265