Do not derive Default on vtable types

#597 (comment)

Author: SimonSapin

src/codegen/mod.rs

@@ -706,11 +706,7 @@ impl<'a> CodeGenerator for Vtable<'a> {
         assert_eq!(item.id(), self.item_id);
         // For now, generate an empty struct, later we should generate function
         // pointers and whatnot.
-        let mut attributes = vec![attributes::repr("C")];
-
-        if ctx.options().derive_default {
-            attributes.push(attributes::derives(&["Default"]))
-        }
+        let attributes = vec![attributes::repr("C")];
 
         let vtable = aster::AstBuilder::new()
             .item()