minor #24766 [FrameworkBundle] Drop serializer.cache option (chalasr)

This PR was merged into the 4.0-dev branch.

Discussion
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[FrameworkBundle] Drop serializer.cache option

| Q             | A
| ------------- | ---
| Branch?       | 4.0
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

BC layer has been removed already but not the option, probably a bad merge.

Commits
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85f09a40f1 [FrameworkBundle] Drop serializer.cache option

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php

@@ -695,9 +695,6 @@ class Configuration implements ConfigurationInterface
                     ->{!class_exists(FullStack::class) && class_exists(Serializer::class) ? 'canBeDisabled' : 'canBeEnabled'}()
                     ->children()
                         ->booleanNode('enable_annotations')->{!class_exists(FullStack::class) && class_exists(Annotation::class) ? 'defaultTrue' : 'defaultFalse'}()->end()
-                        ->scalarNode('cache')
-                            ->setDeprecated('The "%path%.%node%" option is deprecated since Symfony 3.1 and will be removed in 4.0. Configure the "cache.serializer" service under "framework.cache.pools" instead.')
-                        ->end()
                         ->scalarNode('name_converter')->end()
                         ->scalarNode('circular_reference_handler')->end()
                         ->arrayNode('mapping')