minor #29879 Removing isFormValid remnants (yceruto)

This PR was merged into the 4.3-dev branch.

Discussion
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Removing isFormValid remnants

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | not yet
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

See https://travis-ci.org/symfony/symfony/jobs/479399078#L2792

Commits
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53d26e5 Removing isFormValid remnants

src/Symfony/Bundle/FrameworkBundle/Tests/Controller/ControllerTraitTest.php

@@ -569,6 +569,5 @@ trait TestControllerTrait
         createFormBuilder as public;
         getDoctrine as public;
         addLink as public;
-        isFormValid as public;
     }
 }