Remove unused _json_object_node_ensure_field

It is now implemented in Matrix.Event.Base

src/matrix-compacts.vala

@@ -857,40 +857,6 @@ namespace Matrix {
         }
     }
 
-    private Json.Node?
-    _json_object_node_ensure_field(Json.Node node,
-                                   string field_name,
-                                   Json.NodeType field_type)
-        requires(node.get_node_type() == Json.NodeType.OBJECT)
-    {
-        var root = node.get_object();
-        Json.Node? new_node;
-
-        if ((new_node = root.get_member(field_name)) == null) {
-            new_node = new Json.Node(field_type);
-
-            switch (field_type) {
-                case Json.NodeType.OBJECT:
-                    new_node.set_object(new Json.Object());
-
-                    break;
-
-                case Json.NodeType.ARRAY:
-                    new_node.set_array(new Json.Array());
-
-                    break;
-
-                // Other node types don’t need special treatment
-                default:
-                    break;
-            }
-
-            root.set_member(field_name, new_node);
-        }
-
-        return new_node;
-    }
-
     public Json.Node?
     _json_node_deep_copy(Json.Node? node)
     {