minor #1215 Remove duplicate call to add collection (CarsonF)

This PR was merged into the 1.3 branch.

Discussion
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Remove duplicate call to add collection

`doFlush` returns the same `RouteCollection` instance now, so there's no need to add the sub-collection routes to itself.

Commits
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8518a8e doFlush returns the same RouteCollection instance now, so there's no need to add the sub-collection routes to itself

src/Silex/ControllerCollection.php

@@ -210,7 +210,7 @@ private function doFlush($prefix, RouteCollection $routes)
                 $routes->add($name, $controller->getRoute());
                 $controller->freeze();
             } else {
-                $routes->addCollection($controller->doFlush($prefix.$controller->prefix, $routes));
+                $controller->doFlush($prefix.$controller->prefix, $routes);
             }
         }