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_trait_selection/Cargo.toml

@@ -26,6 +26,3 @@ smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 thin-vec = "0.2"
 tracing = "0.1"
 # tidy-alphabetical-end
-
-[lints]
-workspace = true

compiler/rustc_trait_selection/src/lib.rs

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