parse-options: drop parse_opt_unknown_cb()
This low-level callback was introduced in
ce564eb1bd5 (parse-options:
add parse_opt_unknown_cb(), 2016-09-05) so that we could advertise
--indent-heuristic in git-blame's "-h" output, even though the option is
actually handled in parse_revision_opt(). We later stopped doing so in
44ae131e384 (builtin/blame.c: remove '--indent-heuristic' from usage
string, 2019-10-28).
This is a weird thing to do, and in the intervening years, we've never
used it again. Let's drop the helper in the name of simplicity.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>