Revert "Use workspace lints for crates in compiler/ #138084"

Revert <https://github.com/rust-lang/rust/pull/138084> to buy time to
consider options that avoids breaking downstream usages of cargo on
distributed `rustc-src` artifacts, where such cargo invocations fail due
to inability to inherit `lints` from workspace root manifest's
`workspace.lints` (this is only valid for the source rust-lang/rust
workspace, but not really the distributed `rustc-src` artifacts).

This breakage was reported in
<https://github.com/rust-lang/rust/issues/138304>.

This reverts commit 48caf81484, reversing
changes made to c6662879b2.

compiler/rustc_parse/src/lib.rs

@@ -12,6 +12,7 @@
 #![feature(iter_intersperse)]
 #![feature(let_chains)]
 #![feature(string_from_utf8_lossy_owned)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use std::path::{Path, PathBuf};