Don't memoize user.list_groups, Fix #5154

The memoization here was causing incorrect group reports when creating a new user in `user.present.`  Rather than trying to pop the `__context__` under certain conditions, I removed the memoization.  I figure if you're querying a user's group list, you want up-to-date information.  (Also it's not an intensive call to figure out a user's groups, so the optimization is negligible)