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_error_messages/src/lib.rs

@@ -4,7 +4,6 @@
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
 #![feature(type_alias_impl_trait)]
-#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use std::borrow::Cow;