Stabilize Iterator::step_by

Fixes #27741

src/test/run-pass/range_inclusive.rs

@@ -10,8 +10,6 @@
 
 // Test inclusive range syntax.
 
-#![feature(iterator_step_by)]
-
 use std::ops::{RangeInclusive, RangeToInclusive};
 
 fn foo() -> isize { 42 }

src/test/run-pass/sync-send-iterators-in-libcore.rs

@@ -14,7 +14,6 @@
 #![feature(iter_empty)]
 #![feature(iter_once)]
 #![feature(iter_unfold)]
-#![feature(iterator_step_by)]
 #![feature(str_escape)]
 
 use std::iter::{empty, once, repeat};