t7700: drop explicit --no-pack-kept-objects from .keep test

We want to make sure that the default behavior of git-repack,
without any options, continues to treat .keep files as it
always has. Adding an explicit --no-pack-kept-objects, as
ee34a2b did, is a much less interesting test, and prevented
us from noticing the bug fixed by 64d3dc9 (repack: do not
accidentally pack kept objects by default, 2014-06-10).

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