Remove unfixable AsRef FIXMEs.

A long time ago, FIXMEs were left to implement
AsRef and AsRef for all types which implemented
Deref and DerefMut. This wasn't done at the time
because of #23442.

Unfortunately, it's not possible to add these
implementations backwards-compabily, since they
can cause conflicting implementation overlaps.

src/libcore/convert.rs

@@ -382,14 +382,6 @@ impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a mut T where T: AsRef<U>
     }
 }
 
-// FIXME (#23442): replace the above impls for &/&mut with the following more general one:
-// // As lifts over Deref
-// impl<D: ?Sized + Deref, U: ?Sized> AsRef<U> for D where D::Target: AsRef<U> {
-//     fn as_ref(&self) -> &U {
-//         self.deref().as_ref()
-//     }
-// }
-
 // AsMut lifts over &mut
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T: ?Sized, U: ?Sized> AsMut<U> for &'a mut T where T: AsMut<U>
@@ -399,14 +391,6 @@ impl<'a, T: ?Sized, U: ?Sized> AsMut<U> for &'a mut T where T: AsMut<U>
     }
 }
 
-// FIXME (#23442): replace the above impl for &mut with the following more general one:
-// // AsMut lifts over DerefMut
-// impl<D: ?Sized + Deref, U: ?Sized> AsMut<U> for D where D::Target: AsMut<U> {
-//     fn as_mut(&mut self) -> &mut U {
-//         self.deref_mut().as_mut()
-//     }
-// }
-
 // From implies Into
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T, U> Into<U> for T where U: From<T>