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

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

compiler/rustc_lint_defs/src/lib.rs

@@ -1,7 +1,3 @@
-// tidy-alphabetical-start
-#![warn(unreachable_pub)]
-// tidy-alphabetical-end
-
 use rustc_abi::ExternAbi;
 use rustc_ast::AttrId;
 use rustc_ast::attr::AttributeExt;