refs.c: remove fallback-to-main-store code get_submodule_ref_store()

refs.c: remove fallback-to-main-store code get_submodule_ref_store()

At this state, there are three get_submodule_ref_store() callers:

 - for_each_remote_ref_submodule()
 - handle_revision_pseudo_opt()
 - resolve_gitlink_ref()

The first two deal explicitly with submodules (and we should never fall
back to the main ref store as a result). They are only called from
submodule.c:

 - find_first_merges()
 - submodule_needs_pushing()
 - push_submodule()

The last one, as its name implies, deals only with submodules too, and
the "submodule" (path) argument must be a non-NULL, non-empty string.

So, this "if NULL or empty string" code block should never ever
trigger. And it's wrong to fall back to the main ref store
anyway. Delete it.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>