Remove tarsum support for digest package

tarsum is not actually used by the registry. Remove support for it.

Convert numerous uses in unit tests to SHA256.

Update docs to remove mentions of tarsums (which were often inaccurate).

Remove tarsum dependency.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

registry/storage/blobwriter.go

@@ -302,7 +302,7 @@ func (bw *blobWriter) moveBlob(ctx context.Context, desc distribution.Descriptor
 			// get a hash, then the underlying file is deleted, we risk moving
 			// a zero-length blob into a nonzero-length blob location. To
 			// prevent this horrid thing, we employ the hack of only allowing
-			// to this happen for the zero tarsum.
+			// to this happen for the digest of an empty tar.
 			if desc.Digest == digest.DigestSha256EmptyTar {
 				return bw.blobStore.driver.PutContent(ctx, blobPath, []byte{})
 			}