doc/git-repack: don't mention nonexistent "--unpacked" option

doc/git-repack: don't mention nonexistent "--unpacked" option

The documentation for geometric repacking mentions a "--unpacked" option
that supposedly changes how loose objects are rolled up. This option has
never existed, and the implied behaviour, namely to include all unpacked
objects into the resulting packfile, is in fact the default behaviour.

Correct the documentation to not mention this option.

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