Extract method to disconnect users

This lets me override parts of the code to add token-storage without having to copy paste the whole connect logic:

```php
class UserProvider extends FOSUBUserProvider
{
    public function loadUserByOAuthUserResponse(UserResponseInterface $response)
    {
        $user = parent::loadUserByOAuthUserResponse($response);
        $user->setGithubToken($response->getAccessToken());

        return $user;
    }

    public function disconnect(UserInterface $user)
    {
        $user->setGithubToken(null);
        parent::disconnectUser($user);
    }
}
```