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

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

compiler/rustc_hir/src/lib.rs

@@ -13,7 +13,6 @@
 #![feature(never_type)]
 #![feature(rustc_attrs)]
 #![feature(variant_count)]
-#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 extern crate self as rustc_hir;