Simplify doc config

Author: javiereguiluz

.symfony.bundle.yaml

@@ -14,11 +14,6 @@ branches:
 maintained_branches:
     - "2.15.x"
     - "2.16.x"
-doc_dir:
-    "2.12.x": docs/
-    "2.13.x": docs/en/
-    "2.14.x": docs/en/
-    "2.15.x": docs/en/
-    "2.16.x": docs/en/
+doc_dir: "docs/en/"
 current_branch: "2.15.x"
 dev_branch: "2.16.x"