slice_patterns: remove from unstable book

src/doc/unstable-book/src/language-features/slice-patterns.md

@@ -1,32 +0,0 @@
-# `slice_patterns`
-
-The tracking issue for this feature is: [#62254]
-
-[#62254]: https://github.com/rust-lang/rust/issues/62254
-
-------------------------
-
-The `slice_patterns` feature gate lets you use `..` to indicate any number of
-elements inside a pattern matching a slice. This wildcard can only be used once
-for a given array. If there's an pattern before the `..`, the subslice will be
-matched against that pattern. For example:
-
-```rust
-#![feature(slice_patterns)]
-
-fn is_symmetric(list: &[u32]) -> bool {
-    match list {
-        &[] | &[_] => true,
-        &[x, ref inside @ .., y] if x == y => is_symmetric(inside),
-        &[..] => false,
-    }
-}
-
-fn main() {
-    let sym = &[0, 1, 4, 2, 4, 1, 0];
-    assert!(is_symmetric(sym));
-
-    let not_sym = &[0, 1, 7, 2, 4, 1, 0];
-    assert!(!is_symmetric(not_sym));
-}
-```