Drop the old tarsum computation for Docker <0.10

Fixes #375.

This landed in 82021b4 (Merging bypass-tarsum, 2014-04-07, #300), but
those Dockers are now ancient history.  Additional details in the
commit message.  @samalba thought this should land with Docker 1.0
[1], which is a while back now.  @dmp42 recently pushed this back to
docker-registry 1.0 [2].

I'm just floating this now, because I was working on some Storage API
cleanup, and I didn't want to bother with those old code paths.  I'm
happy to just let this PR (and any based on it) cook until the powers
that be feel comfortable dropping support for the older clients.

[1]: https://github.com/docker/docker-registry/issues/375#issuecomment-43799910
[2]: https://github.com/docker/docker-registry/issues/375#issuecomment-50868185