diff: remove lazy config loading

There is no point to this. Either:

1. The program has already loaded git_diff_ui_config, in
which case this is a noop.
2. The program didn't, which means it is plumbing that
does not _want_ git_diff_ui_config to be loaded.

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