minor #20705 [Form] Remove unused var cloner property (ogizanagi)

This PR was merged into the 3.2 branch.

Discussion
----------

[Form] Remove unused var cloner property

| 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

Not used anymore after #19986

EDIT: add missing `use` too.

Commits
-------

0708003 [Form] Remove unused var cloner property

src/Symfony/Component/Form/Extension/DataCollector/FormDataCollector.php

@@ -17,8 +17,10 @@
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\HttpKernel\DataCollector\DataCollector;
 use Symfony\Component\HttpKernel\DataCollector\Util\ValueExporter;
+use Symfony\Component\Validator\ConstraintViolationInterface;
 use Symfony\Component\VarDumper\Caster\Caster;
 use Symfony\Component\VarDumper\Caster\ClassStub;
+use Symfony\Component\VarDumper\Cloner\ClonerInterface;
 use Symfony\Component\VarDumper\Cloner\Data;
 use Symfony\Component\VarDumper\Cloner\Stub;
 use Symfony\Component\VarDumper\Cloner\VarCloner;

src/Symfony/Component/Form/Extension/DataCollector/FormDataExtractor.php

@@ -23,11 +23,6 @@
  */
 class FormDataExtractor implements FormDataExtractorInterface
 {
-    /**
-     * @var VarCloner
-     */
-    private $cloner;
-
     /**
      * Constructs a new data extractor.
      */