doc: Remove the obsolete README.AMCC-eval-boards-cleanup file

The related boards have been removed four years ago already, in
commit 98f705c9 ("powerpc: remove 4xx support"), so this README
file is not required anymore.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Stefan Roese <sr@denx.de>