Don't rebuild rustdoc after checking bootstrap

This works by unconditionally passing -Z unstable-options to the
compiler. This has no affect in practice since bootstrap doesn't use
`deny(rustc::internal)`.

Author: jyn514

src/bootstrap/check.rs

@@ -321,11 +321,9 @@ macro_rules! tool_check_step {
                 }
 
                 // Enable internal lints for clippy and rustdoc
-                // NOTE: this intentionally doesn't enable lints for any other tools,
-                // see https://github.com/rust-lang/rust/pull/80573#issuecomment-754010776
-                if $path == "src/tools/rustdoc" || $path == "src/tools/clippy" {
-                    cargo.rustflag("-Zunstable-options");
-                }
+                // NOTE: this doesn't enable lints for any other tools unless they explicitly add `#![deny(rustc::internal)]`
+                // See https://github.com/rust-lang/rust/pull/80573#issuecomment-754010776
+                cargo.rustflag("-Zunstable-options");
 
                 builder.info(&format!(
                     "Checking stage{} {} artifacts ({} -> {})",