Update AdminSecurityController.php

@rande Hello,

1) I was working on a custom login page and came to compare this login Controller with the latest of FOSUserBundle. This one could be updated to follow those FOSUserBundle commits?
https://github.com/FriendsOfSymfony/FOSUserBundle/commit/20b97f61ede32b37266cb3febf2372f82647b617
https://github.com/FriendsOfSymfony/FOSUserBundle/commit/63e308cad922185fc1dedc9f95e92b23fd1c94b1

2) I removed renderLogin because it seems to no longer have any use to split between 2 methods (correct me if I am wrong ^^) https://github.com/FriendsOfSymfony/FOSUserBundle/blob/master/Controller/SecurityController.php

3) As per the contributing guidelines I would have liked to provide a test but I didn't find an example in the Tests/Controller folder.