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>

docs/storage/cache/redis/redis.go

@@ -249,7 +249,7 @@ func (rsrbds *repositoryScopedRedisBlobDescriptorService) setDescriptor(ctx cont
 	}
 
 	// Also set the values for the primary descriptor, if they differ by
-	// algorithm (ie sha256 vs tarsum).
+	// algorithm (ie sha256 vs sha512).
 	if desc.Digest != "" && dgst != desc.Digest && dgst.Algorithm() != desc.Digest.Algorithm() {
 		if err := rsrbds.setDescriptor(ctx, conn, desc.Digest, desc); err != nil {
 			return err