fix(tvix-glue/fetchers): remove unused allow dead code

This isn't unused anymore, we can remove the
`#![allow(dead_code)]`.

Change-Id: Ia2048394a60161fbfd4ac2a2b64d03350526faa4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11795
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Ilan Joselevich <personal@ilanjoselevich.com>
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/glue/src/fetchers/decompression.rs

@@ -1,5 +1,3 @@
-#![allow(dead_code)] // TODO
-
 use std::{
     io, mem,
     pin::Pin,