Remove two unused feature gates from rustc_query_impl

compiler/rustc_query_impl/src/lib.rs

@@ -3,9 +3,6 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(rustdoc_internals)]
-// this shouldn't be necessary, but the check for `&mut _` is too naive and denies returning a function pointer that takes a mut ref
-#![feature(const_mut_refs)]
-#![feature(const_refs_to_cell)]
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(rustc_attrs)]