Rollup merge of #142700 - theemathas:remove-weak-comment, r=ibraheemdev

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 <#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.

Author: compiler-errors

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,
 }