[Rust] make rust part more idomatic and fix panic of backtrack template (#1370)

* Drop unused variable

* Idiomatic rust

* Fix panic template

codes/rust/chapter_array_and_linkedlist/array.rs

@@ -74,7 +74,6 @@ fn find(nums: &[i32], target: i32) -> Option<usize> {
 fn main() {
     /* 初始化数组 */
     let arr: [i32; 5] = [0; 5];
-    let slice: &[i32] = &[0; 5];
     print!("数组 arr = ");
     print_util::print_array(&arr);
     // 在 Rust 中，指定长度时（[i32; 5]）为数组，不指定长度时（&[i32]）为切片