Refactored FacebookRedirectLoginHelper

I think it's better for the redirect URI to be passed directly to the `getLoginUrl()` (like the `$next` param in `getLogoutUrl()`.
About `getSessionFromRedirect()` usage of `$this->redirectUrl`, it's useless because in that case `redirect_uri` param should always be the current url, because we come from a Facebook redirect that use the previously setted `redirect_uri`.
I've also remove the useless `$state` property.