With get_main_ref_store() being used inside get_ref_store(),
lookup_ref_store() is only used for submodule code path. Rename to
reflect that and delete dead code.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>