minor #22852 [Serializer] Remove a useless legacy annotation (dunglas)

This PR was merged into the 3.3 branch.

Discussion
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[Serializer] Remove a useless legacy annotation

| Q             | A
| ------------- | ---
| Branch?       | 3.3
| Bug fix?      | no
| New feature?  | no <!-- don't forget updating src/**/CHANGELOG.md files -->
| BC breaks?    | no
| Deprecations? | no <!-- don't forget updating UPGRADE-*.md files -->
| Tests pass?   | yes/no
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

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- Features and deprecations must be submitted against the 3.4,
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Commits
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987749b [Serializer] Remove a useless legacy annotation

src/Symfony/Component/Serializer/Tests/Normalizer/AbstractObjectNormalizerTest.php

@@ -26,9 +26,6 @@ class AbstractObjectNormalizerTest extends TestCase
         $this->assertSame('baz', $normalizedData->baz);
     }
 
-    /**
-     * @group legacy
-     */
     public function testInstantiateObjectDenormalizer()
     {
         $data = array('foo' => 'foo', 'bar' => 'bar', 'baz' => 'baz');