fix: build error

```
 Compiling torrust-tracker-contrib-bencode v3.0.0-alpha.12-develop (/home/runner/work/torrust-tracker/torrust-tracker/contrib/bencode)
error: the item `Extend` is imported redundantly
 --> contrib/bencode/src/mutable/encode.rs:1:5
  |
1 | use std::iter::Extend;
  |     ^^^^^^^^^^^^^^^^^
 --> /rustc/2bf78d12d33ae02d10010309a0d85dd04e7cff72/library/std/src/prelude/mod.rs:129:13
  |
  = note: the item `Extend` is already defined here
  |
  = note: `-D unused-imports` implied by `-D unused`
  = help: to override `-D unused` add `#[allow(unused_imports)]`
```

contrib/bencode/src/mutable/encode.rs

@@ -1,5 +1,3 @@
-use std::iter::Extend;
-
 use crate::access::bencode::{BRefAccess, RefKind};
 use crate::access::dict::BDictAccess;
 use crate::access::list::BListAccess;