ref-filter: drop unused atom parameter from get_worktree_path()
The get_worktree_path() function is used to populate the %(worktreepath)
value, but it has never used its "atom" parameter since it was added in
2582083fa1 (ref-filter: add worktreepath atom, 2019-04-28).
Normally we'd use the atom struct to cache any work we do, but in this
case there's a global hashmap that does that caching already. So we can
just drop the unused parameter.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>