minor #36758 [5.1] CS fixes (nicolas-grekas)

This PR was merged into the 5.1-dev branch.

Discussion
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[5.1] CS fixes

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

Commits
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bb164e6615 [5.1] CS fixes

Author: nicolas-grekas

Caster/RdKafkaCaster.php

@@ -11,7 +11,6 @@
 
 namespace Symfony\Component\VarDumper\Caster;
 
-use RdKafka;
 use RdKafka\Conf;
 use RdKafka\Exception as RdKafkaException;
 use RdKafka\KafkaConsumer;