refs: drop infrastructure to peel via iterators

Now that the peeled object ID gets propagated via the `struct reference`
there is no need anymore to call into the reference iterator itself to
dereference an object. Remove this infrastructure.

Most of the changes are straight-forward deletions of code. There is one
exception though in `refs/packed-backend.c::write_with_updates()`. Here
we stop peeling the iterator and instead just pass the peeled object ID
of that iterator directly.

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

refs.h

@@ -1289,10 +1289,6 @@ int repo_migrate_ref_storage_format(struct repository *repo,
  * to the next entry, ref_iterator_advance() aborts the iteration,
  * frees the ref_iterator, and returns ITER_ERROR.
  *
- * The reference currently being looked at can be peeled by calling
- * ref_iterator_peel(). This function is often faster than peel_ref(),
- * so it should be preferred when iterating over references.
- *
  * Putting it all together, a typical iteration looks like this:
  *
  *     int ok;
@@ -1307,9 +1303,6 @@ int repo_migrate_ref_storage_format(struct repository *repo,
  *             // Access information about the current reference:
  *             if (!(iter->flags & REF_ISSYMREF))
  *                     printf("%s is %s\n", iter->refname, oid_to_hex(iter->oid));
- *
- *             // If you need to peel the reference:
- *             ref_iterator_peel(iter, &oid);
  *     }
  *
  *     if (ok != ITER_DONE)
@@ -1400,13 +1393,6 @@ enum ref_iterator_seek_flag {
 int ref_iterator_seek(struct ref_iterator *ref_iterator, const char *refname,
 		      unsigned int flags);
 
-/*
- * If possible, peel the reference currently being viewed by the
- * iterator. Return 0 on success.
- */
-int ref_iterator_peel(struct ref_iterator *ref_iterator,
-		      struct object_id *peeled);
-
 /* Free the reference iterator and any associated resources. */
 void ref_iterator_free(struct ref_iterator *ref_iterator);