removed unused feature imports

src/day2/2.rs

@@ -1,4 +1,3 @@
-#![feature(slice_split_once)]
 #![feature(test)]
 extern crate test;
 

src/day4/1.rs

@@ -1,4 +1,3 @@
-#![feature(slice_split_once)]
 #![feature(test)]
 
 extern crate test;

src/day4/2.rs

@@ -1,5 +1,4 @@
 #![feature(test)]
-#![feature(slice_split_once)]
 
 extern crate test;
 

src/day5/1.rs

@@ -1,4 +1,3 @@
-#![feature(slice_split_once)]
 #![feature(test)]
 
 use std::ops::Range;