Removed chapter from Unstable Book.

src/doc/unstable-book/src/language-features/self-in-typedefs.md

@@ -1,24 +0,0 @@
-# `self_in_typedefs`
-
-The tracking issue for this feature is: [#49303]
-
-[#49303]: https://github.com/rust-lang/rust/issues/49303
-
-------------------------
-
-The `self_in_typedefs` feature gate lets you use the special `Self` identifier
-in `struct`, `enum`, and `union` type definitions.
-
-A simple example is:
-
-```rust
-#![feature(self_in_typedefs)]
-
-enum List<T>
-where
-    Self: PartialOrd<Self> // can write `Self` instead of `List<T>`
-{
-    Nil,
-    Cons(T, Box<Self>) // likewise here
-}
-```