Remove unnecessary T: Clone bound on core::cell::clone_ref.

`clone_ref` does not actually clone `T`,
it just copies a `&T` and increments a borrow count.

Author: SimonSapin

src/libcore/cell.rs

@@ -548,7 +548,7 @@ impl<'b, T: ?Sized> Deref for Ref<'b, T> {
 #[unstable(feature = "core",
            reason = "likely to be moved to a method, pending language changes")]
 #[inline]
-pub fn clone_ref<'b, T:Clone>(orig: &Ref<'b, T>) -> Ref<'b, T> {
+pub fn clone_ref<'b, T>(orig: &Ref<'b, T>) -> Ref<'b, T> {
     Ref {
         _value: orig._value,
         _borrow: orig._borrow.clone(),