helper/schema: Resouce.Diff no longer ResourceProvider API compatible

The old comments said that this interface was API compatible with
terraform.ResourceProvider's Diff method - with the addition of passing
down meta to it, this is no longer the case.

Not too sure if this is really a big deal - schema.Resource never fully
implemented terraform.ResourceProvider, from what I can see, and the
path from Provdier.Diff to Resource.Diff is still pretty clear. Just
wanted to remove an outdated comment.

helper/schema/resource.go

@@ -216,8 +216,7 @@ func (r *Resource) Apply(
 	return r.recordCurrentSchemaVersion(data.State()), err
 }
 
-// Diff returns a diff of this resource and is API compatible with the
-// ResourceProvider interface.
+// Diff returns a diff of this resource.
 func (r *Resource) Diff(
 	s *terraform.InstanceState,
 	c *terraform.ResourceConfig,