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

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

compiler/rustc_trait_selection/src/lib.rs

@@ -31,7 +31,6 @@
 #![feature(unwrap_infallible)]
 #![feature(yeet_expr)]
 #![recursion_limit = "512"] // For rustdoc
-#![warn(unreachable_pub)] // For rustdoc
 // tidy-alphabetical-end
 
 pub mod error_reporting;