Remove TODO that has been implemented. (#193)

internal.Call is exposed by the APICall method.
diff --git a/appengine.go b/appengine.go
index 0cca033..8c96976 100644
--- a/appengine.go
+++ b/appengine.go
@@ -97,8 +97,6 @@
 	return internal.WithContext(parent, req)
 }
 
-// TODO(dsymonds): Add a Call function here? Otherwise other packages can't access internal.Call.
-
 // BlobKey is a key for a blobstore blob.
 //
 // Conceptually, this type belongs in the blobstore package, but it lives in