Remove `Model.__subclass`

Re: #7547, `Model.__subclass()` comes with a lot of surprises and is not very clear. To subclass a model, you need to pass the same name and schema into `mongoose.model()`, but a different collection name. This model then isn't stored in the `mongoose.models` hash, so you have an implicit dangling model. We should consider removing this.

Start by reading issue #7547 and locating the Model.__subclass() entry point and its uses in Mongoose. Determine the compatibility impact of removing it, including the mongoose.model() behavior and dangling-model concern; done means the removal decision and affected behavior are covered consistently.

Written by the indexing model from the issue text.