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_macros/src/lib.rs

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