minor symfony#17395 [DependencyInjection] Remove useless array declaration (paradajozsef)

This PR was merged into the 3.0 branch.

Discussion
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[DependencyInjection] Remove useless array declaration

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

In [2.8](https://github.com/symfony/symfony/blob/2.8/src/Symfony/Component/DependencyInjection/Compiler/CheckReferenceValidityPass.php#L54) ```$ancestors``` was populated with ```$scopes```. 4a3e2f4 removed this, except the array declaration.

Commits
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7240318 [3.0] Remove useless array declaration

Author: Tobion

src/Symfony/Component/DependencyInjection/Compiler/CheckReferenceValidityPass.php

@@ -38,8 +38,6 @@ public function process(ContainerBuilder $container)
     {
         $this->container = $container;
 
-        $ancestors = array();
-
         foreach ($container->getDefinitions() as $id => $definition) {
             if ($definition->isSynthetic() || $definition->isAbstract()) {
                 continue;