Remove unneeded extra prop from Composer.json

Author: thePanz

composer.json

@@ -28,10 +28,5 @@
         "psr-0": {
             "Swift_": "tests/unit"
         }
-    },
-    "extra": {
-        "branch-alias": {
-            "dev-master": "5.4-dev"
-        }
     }
 }