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

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

compiler/rustc_fluent_macro/src/lib.rs

@@ -7,7 +7,6 @@
 #![feature(proc_macro_span)]
 #![feature(rustdoc_internals)]
 #![feature(track_path)]
-#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use proc_macro::TokenStream;