Remove Rc::would_wrap

[unstable, deprecated since 1.15.0]

src/doc/unstable-book/src/SUMMARY.md

@@ -185,7 +185,6 @@
     - [rand](library-features/rand.md)
     - [range_contains](library-features/range-contains.md)
     - [raw](library-features/raw.md)
-    - [rc_would_unwrap](library-features/rc-would-unwrap.md)
     - [retain_hash_collection](library-features/retain-hash-collection.md)
     - [reverse_cmp_key](library-features/reverse-cmp-key.md)
     - [rt](library-features/rt.md)

src/doc/unstable-book/src/library-features/rc-would-unwrap.md

@@ -1,5 +0,0 @@
-# `rc_would_unwrap`
-
-The tracking issue for this feature is: [#28356]
-
-[#28356]: https://github.com/rust-lang/rust/issues/28356