remove compilation warnings

src/main.rs

@@ -1,7 +1,6 @@
 use std::io::{self, Write};
 use std::time::Duration;
 
-use rand::prelude::*;
 use tokio::time::sleep;
 
 static CHARSET: [&str; 221] = [
@@ -103,7 +102,6 @@ impl Cipher {
     }
 
     fn random_unmask(&mut self, num: usize) -> bool {
-        let mut rng = rand::rng();
         let mut idx: Vec<usize> = self
             .mask
             .iter()