refs: remove functions without ref store

refs: remove functions without ref store

The preceding commit has rewritten all callers of ref-related functions
to use the equivalents that accept a `struct ref_store`. Consequently,
the respective variants without the ref store are now unused. Remove
them.

There are likely patch series in-flight that use the now-removed
functions. To help the authors, the old implementations have been added
to "refs.c" in an ifdef'd section as a reference for how to migrate each
of the respective callers.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>