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/paths.go

@@ -396,9 +396,8 @@ type layerLinkPathSpec struct {
 func (layerLinkPathSpec) pathSpec() {}
 
 // blobAlgorithmReplacer does some very simple path sanitization for user
-// input. Mostly, this is to provide some hierarchy for tarsum digests. Paths
-// should be "safe" before getting this far due to strict digest requirements
-// but we can add further path conversion here, if needed.
+// input. Paths should be "safe" before getting this far due to strict digest
+// requirements but we can add further path conversion here, if needed.
 var blobAlgorithmReplacer = strings.NewReplacer(
 	"+", "/",
 	".", "/",
@@ -468,10 +467,6 @@ func (repositoriesRootPathSpec) pathSpec() {}
 //
 // 	<algorithm>/<hex digest>
 //
-// Most importantly, for tarsum, the layout looks like this:
-//
-// 	tarsum/<version>/<digest algorithm>/<full digest>
-//
 // If multilevel is true, the first two bytes of the digest will separate
 // groups of digest folder. It will be as follows:
 //
@@ -494,19 +489,5 @@ func digestPathComponents(dgst digest.Digest, multilevel bool) ([]string, error)
 
 	suffix = append(suffix, hex)
 
-	if tsi, err := digest.ParseTarSum(dgst.String()); err == nil {
-		// We have a tarsum!
-		version := tsi.Version
-		if version == "" {
-			version = "v0"
-		}
-
-		prefix = []string{
-			"tarsum",
-			version,
-			tsi.Algorithm,
-		}
-	}
-
 	return append(prefix, suffix...), nil
 }