Remove extra config

Author: dunglas

composer.json

@@ -15,10 +15,5 @@
     "symfony/security-bundle": "^3.0",
     "symfony/twig-bundle": "^3.0",
     "symfony/validator": "^3.0"
-  },
-  "extra": {
-    "branch-alias": {
-      "dev-master": "2.1.x-dev"
-    }
   }
 }