Remove the CREDITS file

This file is not maintained these days.

We use MAINTAINERS for the maintainership of the supported boards.
For dead boards, we have some clues in doc/README.scrapyard and
also imperishable history in git-log.

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