Remove #![warn(unreachable_pub)] from all compiler/ crates.

It's no longer necessary now that `-Wunreachable_pub` is being passed.

compiler/rustc_transmute/src/lib.rs

@@ -1,6 +1,5 @@
 // tidy-alphabetical-start
 #![feature(never_type)]
-#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 pub(crate) use rustc_data_structures::fx::{FxIndexMap as Map, FxIndexSet as Set};