minor #21618 [FrameworkBundle] Remove unused import (dunglas)

This PR was merged into the 3.2 branch.

Discussion
----------

[FrameworkBundle] Remove unused import

| Q             | A
| ------------- | ---
| Branch?       | 3.2
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Commits
-------

aa5d32b57b [FrameworkBundle] Remove unused import

Author: fabpot

DependencyInjection/FrameworkExtension.php

@@ -35,7 +35,6 @@
 use Symfony\Component\Serializer\Normalizer\DateTimeNormalizer;
 use Symfony\Component\Serializer\Normalizer\JsonSerializableNormalizer;
 use Symfony\Component\Workflow;
-use Symfony\Component\Yaml\Yaml;
 
 /**
  * FrameworkExtension.