Remove last mention of stdsimd

crates/std_detect/src/lib.rs

@@ -20,10 +20,6 @@
 #![cfg_attr(test, allow(unused_imports))]
 #![no_std]
 #![allow(internal_features)]
-// Temporary hack: needed to build against toolchains from before the mass feature renaming.
-// Remove this as soon as the stdarch submodule is updated on nightly.
-#![allow(stable_features)]
-#![cfg_attr(not(feature = "rustc-dep-of-std"), feature(stdsimd))]
 #![cfg_attr(
     all(
         any(target_arch = "x86", target_arch = "x86_64"),