Remove unused feature gates from librustc_mir

Author: bjorn3

src/librustc_mir/lib.rs

@@ -6,21 +6,15 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 
 #![feature(nll)]
 #![feature(in_band_lifetimes)]
-#![feature(inner_deref)]
 #![feature(bool_to_option)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(crate_visibility_modifier)]
-#![feature(core_intrinsics)]
-#![feature(decl_macro)]
 #![feature(drain_filter)]
 #![feature(exhaustive_patterns)]
 #![feature(iter_order_by)]
 #![feature(never_type)]
 #![feature(specialization)]
-#![feature(try_trait)]
-#![feature(unicode_internals)]
-#![feature(slice_concat_ext)]
 #![feature(trusted_len)]
 #![feature(try_blocks)]
 #![feature(associated_type_bounds)]