ath25: drop target

ath25: drop target

This target still only works with kernel 4.14, and not so recent
attempts of getting newer kernel versions supported did not lead
to success. Therefore, drop the target, as we are already two
LTS kernel versions ahead and it does not seem like anybody will
pick up the work.

Patchwork series:
https://patchwork.ozlabs.org/project/openwrt/list/?series=169991&state=*

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>