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

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

compiler/rustc_lexer/src/lib.rs

@@ -23,7 +23,6 @@
 // We want to be able to build this crate with a stable compiler,
 // so no `#![feature]` attributes should be added.
 #![deny(unstable_features)]
-#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 mod cursor;