Delete deprecated & unstable range-specific step_by

Replacement: 41439
Deprecation: 42310 for 1.19
Fixes 41477

src/test/run-pass/range_inclusive.rs

@@ -10,7 +10,7 @@
 
 // Test inclusive range syntax.
 
-#![feature(inclusive_range_syntax, inclusive_range, step_by)]
+#![feature(inclusive_range_syntax, inclusive_range, iterator_step_by)]
 
 use std::ops::{RangeInclusive, RangeToInclusive};
 

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

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