ath25: drop target

ath25: drop target

ath25 has been on life support for the last couple of releases, eventually
leading to marking it as source-only in 2023.

It has been basically only touched to do a kernel bump so that we can make
the new OpenWrt release which was a challenge due to small RAM amount.

However, with the attempt of kernel 6.1 update it turns out that kernel
cannot even finish booting due to RAM constraints, so its time to let this
target go.

Signed-off-by: Robert Marko <robimarko@gmail.com>