check-docs: drop git-help special-case

check-docs: drop git-help special-case

The check-docs target special-cases git-help to avoid
mentioning it as "documented but removed". This dates back
to the early implementation of git-help, when its code was
simply included inside git.c.

These days it is a full-fledged builtin (in builtin/help.c)
and does not need special-casing.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>