Remove use of request info to identify user in AuthComponent::login().

Your login action would now be similar to:
```php
...
if ($this->request->is('post')) {
    $user = $this->Auth->identify();
    if ($user) {
        $this->Auth->login($user);
        // redirect somewhere
    }
    // set error flash
}
...
```
I like `setUser()`.