test-ref-store: drop unimplemented reflog-expire command

test-ref-store: drop unimplemented reflog-expire command

The reflog-expire command has been unimplemented since it was added in
80f2a6097c (t/helper: add test-ref-store to test ref-store functions,
2017-03-26). This causes -Wunused-parameter to complain, since the
function just calls die() without looking at its arguments.

We could mark these as UNUSED to silence the warning. But let's just
drop the function. It has no callers in the test suite and is not doing
anything useful, beyond perhaps reminding us that it's something we
_could_ be testing.

But since the bulk of the work in adding such tests would be the shell
bits that actually examine the reflog state before and after expiration,
this is not even a useful step in that direction. Somebody who wants to
do that work later can easily add this function back.

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