lantiq-xrx200: remove obsolete dsl-app again

ltq-vdsl-app was renamed to ltq-vdsl-vr9-app
https://github.com/openwrt/openwrt/commit/33e2115fe449d1b9e97ce8c5be96f580442fd3d9
ltq-dsl-base is a new package that was added with 23.05