slice_patterns is stable now

tests/run-pass/issue-15080.rs

@@ -1,5 +1,3 @@
-#![feature(slice_patterns)]
-
 fn main() {
     let mut x: &[_] = &[1, 2, 3, 4];
 

tests/run-pass/issue-17877.rs

@@ -1,5 +1,3 @@
-#![feature(slice_patterns)]
-
 fn main() {
     assert_eq!(match [0u8; 16*1024] {
         _ => 42_usize,

tests/run-pass/subslice_array.rs

@@ -1,5 +1,3 @@
-#![feature(slice_patterns)]
-
 fn bar(a: &'static str, b: &'static str) -> [&'static str; 4] {
     [a, b, b, a]
 }

tests/run-pass/vec-matching-fold.rs

@@ -1,5 +1,3 @@
-#![feature(slice_patterns)]
-
 use std::fmt::Debug;
 
 fn foldl<T, U, F>(values: &[T],