remove allow(dead_code) leftovers from serial/parallel compiler

compiler/rustc_data_structures/src/sync/lock.rs

@@ -1,8 +1,6 @@
 //! This module implements a lock which only uses synchronization if `might_be_dyn_thread_safe` is true.
 //! It implements `DynSend` and `DynSync` instead of the typical `Send` and `Sync` traits.
 
-#![allow(dead_code)]
-
 use std::fmt;
 
 #[derive(Clone, Copy, PartialEq)]

compiler/rustc_data_structures/src/sync/parallel.rs

@@ -1,8 +1,6 @@
 //! This module defines parallel operations that are implemented in
 //! one way for the serial compiler, and another way the parallel compiler.
 
-#![allow(dead_code)]
-
 use std::any::Any;
 use std::panic::{AssertUnwindSafe, catch_unwind, resume_unwind};