Remove three const_ features

Summary: Not needed or already stable.

Reviewed By: bobyangyf

Differential Revision: D40893884

fbshipit-source-id: edbbd09a1d011ddbce09cf3bfbc5ad3b47570fbd

Author: stepancheg

starlark/src/lib.rs

@@ -350,11 +350,8 @@
 #![feature(bench_black_box)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
-#![feature(const_fn_fn_ptr_basics)]
-#![feature(const_fn_trait_bound)]
 #![feature(const_mut_refs)]
 #![feature(const_ptr_offset)]
-#![feature(const_refs_to_cell)]
 #![feature(const_type_id)]
 #![feature(core_intrinsics)]
 #![feature(generic_associated_types)]