remove use std::ascii::AsciiExt for Rust 1.23

exercises/atbash-cipher/example.rs

@@ -1,5 +1,3 @@
-use std::ascii::AsciiExt;
-
 fn ascii(ch: char) -> u8 {
     ch as u8
 }

exercises/crypto-square/example.rs

@@ -1,5 +1,3 @@
-use std::ascii::AsciiExt;
-
 extern crate itertools;
 use itertools::Itertools;
 

exercises/etl/example.rs

@@ -1,4 +1,3 @@
-use std::ascii::AsciiExt;
 use std::collections::BTreeMap;
 
 pub fn transform(input: &BTreeMap<i32, Vec<char>>) -> BTreeMap<char, i32> {

exercises/forth/example.rs

@@ -1,4 +1,3 @@
-use std::ascii::AsciiExt;
 use std::collections::HashMap;
 use std::collections::LinkedList;
 use std::str::FromStr;

exercises/pangram/example.rs

@@ -1,5 +1,4 @@
 use std::collections::BTreeSet;
-use std::ascii::AsciiExt;
 use std::iter::FromIterator;
 
 pub fn is_pangram(sentence: &str) -> bool {