Remove the custom impl of AttrsOwner for ImplItem

The default impl should have the same behaviour, and it can be generated
by codegen.
See also `ModuleItem` and `NominalDef`

crates/ra_syntax/src/ast/generated.rs

@@ -1298,6 +1298,7 @@ impl AstNode for ImplItem {
         }
     }
 }
+impl ast::AttrsOwner for ImplItem {}
 impl ImplItem {}
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct ImplTraitType {