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

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

compiler/rustc_query_system/src/lib.rs

@@ -6,7 +6,6 @@
 #![feature(hash_raw_entry)]
 #![feature(let_chains)]
 #![feature(min_specialization)]
-#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 pub mod cache;