drop pure pass-through config callbacks

drop pure pass-through config callbacks

Commit fd2d4c135e (gpg-interface: lazily initialize and read the
configuration, 2023-02-09) shrunk a few custom config callbacks so that
they are just one-liners of:

  return git_default_config(...);

We can drop them entirely and replace them direct calls of
git_default_config() intead. This makes the code a little shorter and
easier to understand (with the downside being that if they do grow
custom options again later, we'll have to recreate the functions).

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