Remove box-syntax from unstable book

src/doc/unstable-book/src/language-features/box-syntax.md

@@ -1,22 +0,0 @@
-# `box_syntax`
-
-The tracking issue for this feature is: [#49733]
-
-[#49733]: https://github.com/rust-lang/rust/issues/49733
-
-See also [`box_patterns`](box-patterns.md)
-
-------------------------
-
-Currently the only stable way to create a `Box` is via the `Box::new` method.
-Also it is not possible in stable Rust to destructure a `Box` in a match
-pattern. The unstable `box` keyword can be used to create a `Box`. An example
-usage would be:
-
-```rust
-#![feature(box_syntax)]
-
-fn main() {
-    let b = box 5;
-}
-```