Remove double registration of the event listeners

Composer already registers listeners when the plugin class implements EventSubscriberInterface

src/PackageVersions/Installer.php

@@ -57,7 +57,7 @@ public static function getVersion(string $packageName) : string
      */
     public function activate(Composer $composer, IOInterface $io)
     {
-        $composer->getEventDispatcher()->addSubscriber($this);
+        // Nothing to do here, as all features are provided through event listeners
     }
 
     /**