t/helper: drop pack-refs wrapper

t/helper: drop pack-refs wrapper

The test helper provides a "ref-store <store> pack-refs" wrapper that
more or less directly invokes `refs_pack_refs()`. This helper is only
used in a single test with the "PACK_REFS_PRUNE" and "PACK_REFS_ALL"
flags. Both of these flags can directly be accessed via git-pack-refs(1)
though via the `--all` and `--prune` flags, which makes the helper
superfluous.

Refactor the test to use git-pack-refs(1) instead of the test helper.
Drop the now-unused test helper command.

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