minor #20185 Remove unused variable (lin-lu)

This PR was submitted for the master branch but it was merged into the 2.8 branch instead (closes #20185).

Discussion
----------

Remove unused variable

Remove unused variable $key.

Commits
-------

129694f Remove unused variable

src/Symfony/Component/EventDispatcher/EventDispatcher.php

@@ -92,7 +92,7 @@ class EventDispatcher implements EventDispatcherInterface
         }
 
         foreach ($this->listeners[$eventName] as $priority => $listeners) {
-            if (false !== ($key = array_search($listener, $listeners, true))) {
+            if (false !== array_search($listener, $listeners, true)) {
                 return $priority;
             }
         }