refactor ~ remove unneeded mem::uninitialized() code

.# [why]

`mem::ununitialized()` is deprecated as of rust v1.39.0.

And this use is likely a premature optimization attempting to avoid
initializing the byte array to 0, which is usually a *very* fast operation.

* ref: <https://github.com/rust-lang/rust/blob/master/RELEASES.md>
* ref: <https://stackoverflow.com/questions/3654905/faster-way-to-zero-memory-than-with-memset>

src/cksum/cksum.rs

@@ -14,8 +14,6 @@ extern crate uucore;
 
 use std::fs::File;
 use std::io::{self, stdin, BufReader, Read};
-#[cfg(not(windows))]
-use std::mem;
 use std::path::Path;
 
 include!(concat!(env!("OUT_DIR"), "/crc_table.rs"));
@@ -39,16 +37,10 @@ fn crc_final(mut crc: u32, mut length: usize) -> u32 {
     !crc
 }
 
-#[cfg(windows)]
 fn init_byte_array() -> Vec<u8> {
     vec![0; 1024 * 1024]
 }
 
-#[cfg(not(windows))]
-fn init_byte_array() -> [u8; 1024 * 1024] {
-    unsafe { mem::uninitialized() }
-}
-
 #[inline]
 fn cksum(fname: &str) -> io::Result<(u32, usize)> {
     let mut crc = 0u32;