fetch: drop unused DISPLAY_FORMAT_UNKNOWN enum value

fetch: drop unused DISPLAY_FORMAT_UNKNOWN enum value

With 50957937f9 (fetch: introduce `display_format` enum, 2023-05-10), a
new enumeration was introduced to determine the display format that is
to be used by git-fetch(1). The `DISPLAY_FORMAT_UNKNOWN` value isn't
ever used though, and neither do we rely on the explicit `0` value for
initialization anywhere.

Remove the enum value.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>