csum-file: use idempotent git_hash_discard()

csum-file: use idempotent git_hash_discard()

Now that it is safe to call git_hash_discard() even after finalizing it,
we can simplify our cleanup logic a bit. This is mostly undoing a few
bits of 64337aecde (csum-file: always finalize or discard hash,
2026-07-02):

  - We no longer need a separate free_hashfile_memory() function for
    finalize_hashfile(). It can just call free_hashfile(), which will
    now discard (or not) the hash as appropriate.

  - When f->skip_hash is set, we don't need to discard; we can rely on
    free_hashfile() to do it.

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