ath79: remove obsolete pci-ath9k-fixup.[c,h] files

This was old code from the AR71XXs target days that
doesn't get compiled and used anymore.

Bringing up AR92xx and earlier chips from their
OWL-Emulator state is currently done by the upstream
ath9k-pci-owl-loader module. (see the kmod-owl-loader
package).

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>