Disable docs for compiler-builtins and sysroot

Bootstrap already had a manual doc filter for the `sysroot` crate, but
other library crates keep themselves out of the public docs by setting
`[lib] doc = false` in their manifest. This seems like a better solution
to hide `compiler-builtins` docs, and removes the `sysroot` hack too.

src/bootstrap/src/core/build_steps/doc.rs

@@ -739,10 +739,6 @@ fn doc_std(
     }
 
     for krate in requested_crates {
-        if krate == "sysroot" {
-            // The sysroot crate is an implementation detail, don't include it in public docs.
-            continue;
-        }
         cargo.arg("-p").arg(krate);
     }