Fixed compiler warnings

src/solutions/day04.rs

@@ -1,4 +1,5 @@
-use std::{borrow::BorrowMut, collections::HashMap, fs::copy, vec};
+use std::collections::HashMap;
+
 
 use super::Solution;
 
@@ -30,7 +31,7 @@ impl Solution for Day04 {
         input: &mut Vec<String>,
     ) -> Result<Box<dyn std::fmt::Display>, Box<dyn std::error::Error>> {
         // Remove Card XXX: at start and append card index
-        let mut cards: Vec<(usize, &str)> = input
+        let cards: Vec<(usize, &str)> = input
             .iter()
             .map(|c| c.split(":").into_iter().last().unwrap())
             .into_iter()