The raw mongoAuth() interface in the JS driver understands a three arg call as mongo.auth(db, user, pass) which is hardcoded to MONGODB-CR.

In practice this is never reached since db.js normalizes calls to db.auth() into just the one (object) or two (string,string) arg forms (the latter being translated into the former by the JS helper code).

This diff removes the dead code path and simplifies the C++/JS connector.