minor #5970 Remove isSubmitted call (DanielSiepmann)

This PR was submitted for the 3.0 branch but it was merged into the 2.3 branch instead (closes #5970).

Discussion
----------

Remove isSubmitted call

Streamline with http://symfony.com/doc/current/book/forms.html#handling-form-submissions
There is no need for `isSubmitted` as this is done inside `isValid`

Commits
-------

cebd5fd Remove isSubmitted call

Author: wouterj

cookbook/doctrine/registration_form.rst

@@ -222,7 +222,7 @@ controller for displaying the registration form::
 
             // 2) handle the submit (will only happen on POST)
             $form->handleRequest($request);
-            if ($form->isValid() && $form->isSubmitted()) {
+            if ($form->isSubmitted() && $form->isValid()) {
                 // 3) Encode the password (you could also do this via Doctrine listener)
                 $encoder = $this->get('security.encoder_factory')
                     ->getEncoder($user);