powerpc: mpc8260ads: remove orphan board

This board has been orphan for a while.
(Emails to its maintainer have been bouncing.)

Because MPC82xx family is old enough, nobody would pick up
the maintainership on it.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Wolfgang Denx <wd@denx.de>