minor #37236 remove unused param from validator service config (Tobion)

This PR was merged into the 4.4 branch.

Discussion
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remove unused param from validator service config

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       |
| License       | MIT
| Doc PR        |

Commits
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d75ec21 remove unused param from validator service config

src/Symfony/Bundle/FrameworkBundle/Resources/config/validator.xml

@@ -5,7 +5,6 @@
     xsi:schemaLocation="http://symfony.com/schema/dic/services https://symfony.com/schema/dic/services/services-1.0.xsd">
 
     <parameters>
-        <parameter key="validator.mapping.cache.prefix" />
         <parameter key="validator.mapping.cache.file">%kernel.cache_dir%/validation.php</parameter>
     </parameters>