slice_patterns: adjust error codes

Author: Centril

src/librustc_error_codes/error_codes/E0527.md

@@ -17,8 +17,6 @@ Ensure that the pattern is consistent with the size of the matched
 array. Additional elements can be matched with `..`:
 
 ```
-#![feature(slice_patterns)]
-
 let r = &[1, 2, 3, 4];
 match r {
     &[a, b, ..] => { // ok!

src/librustc_error_codes/error_codes/E0528.md

@@ -4,8 +4,6 @@ matched array.
 Example of erroneous code:
 
 ```compile_fail,E0528
-#![feature(slice_patterns)]
-
 let r = &[1, 2];
 match r {
     &[a, b, c, rest @ ..] => { // error: pattern requires at least 3
@@ -19,8 +17,6 @@ Ensure that the matched array has at least as many elements as the pattern
 requires. You can match an arbitrary number of remaining elements with `..`:
 
 ```
-#![feature(slice_patterns)]
-
 let r = &[1, 2, 3, 4, 5];
 match r {
     &[a, b, c, rest @ ..] => { // ok!

src/librustc_error_codes/error_codes/E0730.md

@@ -18,8 +18,6 @@ Ensure that the pattern is consistent with the size of the matched
 array. Additional elements can be matched with `..`:
 
 ```
-#![feature(slice_patterns)]
-
 let r = &[1, 2, 3, 4];
 match r {
     &[a, b, ..] => { // ok!