Before we can implement a `UseLinq3` switch in `MongoCollectionSettings` we need the LINQ2 and LINQ3 versions of `AsQueryable` to both return the same `IMongoQueryable<T>` type (they will of course return different implementations of that interface).

To accomplish this remove the `IMongoQueryable` interface in LINQ3 and only use one interface shared between LINQ2 and LINQ3.