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

(Except for `rustc_codegen_cranelift`.)

It's no longer necessary now that `unreachable_pub` is in the workspace
lints.

compiler/rustc_serialize/src/lib.rs

@@ -14,7 +14,6 @@
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(rustdoc_internals)]
-#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 pub use self::serialize::{Decodable, Decoder, Encodable, Encoder};