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/spec/manifest-v2-1.md

@@ -55,8 +55,8 @@ Manifest provides the base accessible fields for working with V2 image format
    An fsLayer is a struct consisting of the following fields
       - **`blobSum`** *digest.Digest*
       
-      blobSum is the digest of the referenced filesystem image layer.  A 
-      digest can be a tarsum or sha256 hash.
+      blobSum is the digest of the referenced filesystem image layer. A 
+      digest must be a sha256 hash.
  
    
 - **`history`** *array*