auth: turn dead AuthUser code into assertion

The result of db.User.get_dict never contains the keys 'api_keys' or
'permissions'. The keys returned by get_dict are 1) all the User table
columns, 2) the keys explicitly defined in User.__json__, and 3) the
keys defined in User.get_api_data, none of which include the two
blacklisted keys.

'api_keys' would be returned if __json__ called get_api_data with
argument details=True; but currently that is not the case.

In case there's a reason why these two keys must never appear in an
AuthUser object, the check has not been removed entirely; instead, it's
been turned into an assertion. This way, it will be noticed if __json__
is later modified to request detailed API data, for instance.