don't strip new-computeds from plan diffs

Stripping these was a patch for some provider behavior which was fixed
in other ways, and is no longer needed.
Removing this allows us to implement correct CusomizeDiffFuncs in
providers so that they can mark fields with empty values as computed
during a plan.

helper/plugin/grpc_provider.go

@@ -611,17 +611,6 @@ func (s *GRPCProviderServer) PlanResourceChange(_ context.Context, req *proto.Pl
 		priorState = &terraform.InstanceState{}
 	}
 
-	// if we're not creating a new resource, remove any new computed fields
-	if !create {
-		for attr, d := range diff.Attributes {
-			// If there's no change, then don't let this go through as NewComputed.
-			// This usually only happens when Old and New are both empty.
-			if d.NewComputed && d.Old == d.New {
-				delete(diff.Attributes, attr)
-			}
-		}
-	}
-
 	// now we need to apply the diff to the prior state, so get the planned state
 	plannedAttrs, err := diff.Apply(priorState.Attributes, res.CoreConfigSchemaWhenShimmed())
 	schema.FixupAsSingleInstanceStateOut(