fix(dgraph): Do not return reverse edges from expandEdges (#7461)

We don't need to return reverse edges when we expand the types.
When alpha is running in strict mutations mode and if we try to
delete a node with a type that contains a reverse edge then that
could cause the deletion to fail because the type would be expanded
with the reverse edges.

With this PR, we don't process the reverse edges when processing a type.
Fixes DGRAPH-3057

query/mutation.go

@@ -112,6 +112,10 @@ func expandEdges(ctx context.Context, m *pb.Mutations) ([]*pb.DirectedEdge, erro
 		}
 
 		for _, pred := range preds {
+			// Do not return reverse edges.
+			if x.ParseAttr(pred)[0] == '~' {
+				continue
+			}
 			edgeCopy := *edge
 			edgeCopy.Attr = pred
 			edges = append(edges, &edgeCopy)