parse_color: drop COLOR_BACKGROUND macro

Commit 695d95d (parse_color: refactor color storage,
2014-11-20) introduced two macros, COLOR_FOREGROUND and
COLOR_BACKGROUND. The latter conflicts with a system macro
defined on Windows, breaking compilation there.

The simplest solution is to just get rid of these macros
entirely. They are constants that are only used in one place
(since the whole point of 695d95d was to avoid repeating
ourselves). Their main function is to make the magic
character constants more readable, but we can do the same
thing with a comment.

Reported-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>