ar71xx: drop target

This target has been mostly replaced by ath79 and won't be included
in the upcoming release anymore. Finally put it to rest.

This also removes all references in packages, tools, etc. as well as
the uboot-ar71xx and vsc73x5-ucode packages.

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