Update stdarch README

Author: Kobzol

library/stdarch/README.md

@@ -4,15 +4,7 @@ stdarch - Rust's standard library SIMD components
 [![Actions Status](https://github.com/rust-lang/stdarch/workflows/CI/badge.svg)](https://github.com/rust-lang/stdarch/actions)
 
 
-# Crates
-
-This repository contains two main crates:
-
-* [`core_arch`](crates/core_arch/README.md) implements `core::arch` - Rust's
-  core library architecture-specific intrinsics, and
+This repository contains the [`core_arch`](crates/core_arch/README.md) crate, which implements `core::arch` - Rust's core library architecture-specific intrinsics.
 
-* [`std_detect`](crates/std_detect/README.md) implements `std::detect` - Rust's
-  standard library run-time CPU feature detection.
-
 The `std::simd` component now lives in the
 [`packed_simd_2`](https://github.com/rust-lang/packed_simd) crate.