Remove incorrect comments in Weak

It is currently possible to create a dangling `Weak` to a DST by
calling `Weak::new()` for a sized type, then doing an unsized coercion.
Therefore, the comments are wrong.

These comments were added in <https://github.com/rust-lang/rust/pull/73845>.
As far as I can tell, the guarantee in the comment was only previously
used in the `as_ptr` method. However, the current implementation of
`as_ptr` no longer relies on this guarantee.

library/alloc/src/rc.rs

@@ -3004,7 +3004,6 @@ pub struct Weak<
     // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
     // to allocate space on the heap. That's not a value a real pointer
     // will ever have because RcInner has alignment at least 2.
-    // This is only possible when `T: Sized`; unsized `T` never dangle.
     ptr: NonNull<RcInner<T>>,
     alloc: A,
 }

library/alloc/src/sync.rs

@@ -342,7 +342,6 @@ pub struct Weak<
     // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
     // to allocate space on the heap. That's not a value a real pointer
     // will ever have because RcInner has alignment at least 2.
-    // This is only possible when `T: Sized`; unsized `T` never dangle.
     ptr: NonNull<ArcInner<T>>,
     alloc: A,
 }