Remove lint annotations in specific crates that are already enforced by rustbuild

Remove some random unnecessary lint `allow`s

src/bootstrap/bin/rustdoc.rs

@@ -2,7 +2,8 @@
 //!
 //! See comments in `src/bootstrap/rustc.rs` for more information.
 
-#![deny(warnings)]
+// NO-RUSTC-WRAPPER
+#![deny(warnings, rust_2018_idioms)]
 
 use std::env;
 use std::process::Command;