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_index/Cargo.toml

@@ -21,6 +21,3 @@ nightly = [
 ]
 rustc_randomized_layouts = []
 # tidy-alphabetical-end
-
-[lints]
-workspace = true

compiler/rustc_index/src/lib.rs

@@ -4,6 +4,7 @@
 #![cfg_attr(feature = "nightly", feature(extend_one, step_trait, test))]
 #![cfg_attr(feature = "nightly", feature(new_range_api))]
 #![cfg_attr(feature = "nightly", feature(new_zeroed_alloc))]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 pub mod bit_set;