[8.x] Guess database factory model by default

[nunomaduro]

At this time, database factories have this hidden feature where database models can be "guessed". As example, the following code just works:

class User extends Model
{
    use HasFactory;
}

class UserFactory extends Factory
{
    public function definition()
    {
        return [
            // ..
        ];
    }
}

User::factory()->create(); // Guesses UserFactory
UserFactory::new()->create(); // Guesses User

So, this pull request proposes that remove protected $model from the Factory stub, as probably the current "guess" logic works for 99.99% of the people. In addition, I've also pull requested to the skeleton that we remove protected $model = User::class from the UserFactory.php: laravel/laravel#5713.