Abolish Model::firstByAttributes

Since we now support [array wheres](https://github.com/laravel/framework/commit/87b267a232983abdac7c23c2dc6b1b270dd24b8a), there's no reason for the `firstByAttributes` method anymore. It's kind of a mouthful, and not even clearer or shorter:

    Model::firstByAttributes($attributes);
    Model::where($attributes)->first();

We might even be able to get rid of the method completely, since it was never a `public` method; I'm not sure about the policy regarding `protected` methods, since people might be using it from within one of their models.

I left it in there for now, but if you think we can get rid of it...