for-each-ref: remove unused variable

No code ever used this symbol since the command was introduced at
9f613ddd (Add git-for-each-ref: helper for language bindings,
2006-09-15).

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>