ceph-object-corpus: prune some old releases

We do not need to test these old object encodings as they are covered by
later object collections.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1286 from ceph/wip-corpus

ceph-object-corpus: prune some old releases

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>