bug #15895 [Security] Allow user providers to be defined in many files (lyrixx)

This PR was merged into the 2.3 branch.

Discussion
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[Security] Allow user providers to be defined in many files

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #15894
| License       | MIT
| Doc PR        | -

Commits
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9165191 [Security] Allow user providers to be defined in many files

src/Symfony/Bundle/SecurityBundle/DependencyInjection/MainConfiguration.php

@@ -321,7 +321,6 @@ class MainConfiguration implements ConfigurationInterface
                         ),
                         'my_entity_provider' => array('entity' => array('class' => 'SecurityBundle:User', 'property' => 'username')),
                     ))
-                    ->disallowNewKeysInSubsequentConfigs()
                     ->isRequired()
                     ->requiresAtLeastOneElement()
                     ->useAttributeAsKey('name')