Remove FnBox specialization of impl FnOnce for Box<impl FnOnce>.

Author: qnighy

src/liballoc/boxed.rs

@@ -700,7 +700,7 @@ impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
 impl<A, F: FnOnce<A> + ?Sized> FnOnce<A> for Box<F> {
     type Output = <F as FnOnce<A>>::Output;
 
-    default extern "rust-call" fn call_once(self, args: A) -> Self::Output {
+    extern "rust-call" fn call_once(self, args: A) -> Self::Output {
         <F as FnOnce<A>>::call_once(*self, args)
     }
 }
@@ -777,22 +777,6 @@ impl<A, F> FnBox<A> for F
     }
 }
 
-#[unstable(feature = "fnbox",
-           reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
-impl<A, R> FnOnce<A> for Box<dyn FnBox<A, Output = R> + '_> {
-    extern "rust-call" fn call_once(self, args: A) -> R {
-        self.call_box(args)
-    }
-}
-
-#[unstable(feature = "fnbox",
-           reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
-impl<A, R> FnOnce<A> for Box<dyn FnBox<A, Output = R> + Send + '_> {
-    extern "rust-call" fn call_once(self, args: A) -> R {
-        self.call_box(args)
-    }
-}
-
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}