fixup-builtins: retire an old transition helper script

This script was added in 36e5e70 (Start deprecating "git-command" in
favor of "git command", 2007-06-30) with the intent of aiding the
transition away from dashed forms.

It has already been used to help the transision and served its
purpose, and is no longer very useful for follow-up work, because
the majority of remaining matches it finds are false positives.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>