[U-Boot] [RFC] Removal of CREDITS

Hi.

The "CREDITS" file at the top directory does not seem to be used these days.

The contents are mostly credits of old board supports.

We use MAINTAINERS for the maintainership of the supported boards.

For dead boards, we have clues in doc/README.scrapyard and also imperishable history in git-log.

I think we do not need the CREDITS file any more.

Comments are welcome.

Best Regards Masahiro Yamada