Remove unused features from rustc_middle.

Author: nnethercote

compiler/rustc_middle/src/lib.rs

@@ -37,7 +37,6 @@
 #![feature(box_as_ptr)]
 #![feature(box_patterns)]
 #![feature(closure_track_caller)]
-#![feature(const_type_name)]
 #![feature(core_intrinsics)]
 #![feature(coroutines)]
 #![feature(debug_closure_helpers)]
@@ -50,7 +49,6 @@
 #![feature(intra_doc_pointers)]
 #![feature(iter_from_coroutine)]
 #![feature(let_chains)]
-#![feature(macro_metavar_expr)]
 #![feature(min_specialization)]
 #![feature(negative_impls)]
 #![feature(never_type)]