Remove branch-version (keep them for contracts only)

Author: nicolas-grekas

composer.json

@@ -34,8 +34,5 @@
         "symfony/monolog-bridge": "For using the log server.",
         "symfony/expression-language": "For using the filter option of the log server."
     },
-    "minimum-stability": "dev",
-    "extra": {
-        "branch-version": "3.4"
-    }
+    "minimum-stability": "dev"
 }