Simplify the stdbuild section

Found this when encountering the code in the rustc submodule and changing the allow for the warnings to deny.

* `no_std` is stable so it does not have to be listed in the `feature` attribute
* `no_std` as an attribute for the crate is already implied by the `#![cfg_attr(not(feature = "use_std"), no_std)]` below
* `staged_api` as an attribute gives a warning. That also matches my knowledge.