remove useless #[allow] in a test

Author: WaffleLapkin

src/test/ui/issues/issue-30371.rs

@@ -1,6 +1,5 @@
 // run-pass
 #![allow(unreachable_code)]
-#![allow(unused_mut)] // rust-lang/rust#54586
 #![deny(unused_variables)]
 
 fn main() {