vinyl: simplify cache iterator restore

Since ->restore() is not used by the read iterator to start iteration
any more, we can remove the corresponding code from the cache iterator
->restore() callback. Although it might be tempting to simplify it even
more by doing a full lookup every time the cache version changes, as we
already do in case of memory and txw iterators, it doesn't seem to be a
sound idea, because the read iterator itself can change the cache
version on each iteration by inserting new elements into the cache, even
if there were no disk accesses.