Remove stability annotations from trait impl items

Remove `stable` stability annotations from inherent impls

src/liballoc/arc.rs

@@ -881,7 +881,6 @@ impl<T> fmt::Pointer for Arc<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Default> Default for Arc<T> {
-    #[stable(feature = "rust1", since = "1.0.0")]
     fn default() -> Arc<T> {
         Arc::new(Default::default())
     }