Remove bench_black_box from the list of unstable features

Author: chriswailes

Diff for: crates/core_arch/src/lib.rs

@@ -31,7 +31,6 @@
     f16c_target_feature,
     allow_internal_unstable,
     decl_macro,
-    bench_black_box,
     asm_const,
     target_feature_11
 )]

Diff for: crates/stdarch-test/src/lib.rs

@@ -3,7 +3,6 @@
 //! This basically just disassembles the current executable and then parses the
 //! output once globally and then provides the `assert` function which makes
 //! assertions about the disassembly of a function.
-#![feature(bench_black_box)] // For black_box
 #![deny(rust_2018_idioms)]
 #![allow(clippy::missing_docs_in_private_items, clippy::print_stdout)]