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_mir_build/Cargo.toml

@@ -28,6 +28,3 @@ rustc_span = { path = "../rustc_span" }
 rustc_trait_selection = { path = "../rustc_trait_selection" }
 tracing = "0.1"
 # tidy-alphabetical-end
-
-[lints]
-workspace = true

compiler/rustc_mir_build/src/lib.rs

@@ -8,6 +8,7 @@
 #![feature(if_let_guard)]
 #![feature(let_chains)]
 #![feature(try_blocks)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 // The `builder` module used to be named `build`, but that was causing GitHub's