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

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

compiler/rustc_builtin_macros/src/lib.rs

@@ -18,7 +18,6 @@
 #![feature(rustdoc_internals)]
 #![feature(string_from_utf8_lossy_owned)]
 #![feature(try_blocks)]
-#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 extern crate proc_macro;