Remove unused features.

compiler/rustc_mir_transform/src/lib.rs

@@ -3,13 +3,11 @@
 #![feature(box_patterns)]
 #![feature(const_type_name)]
 #![feature(cow_is_borrowed)]
-#![feature(decl_macro)]
 #![feature(if_let_guard)]
 #![feature(impl_trait_in_assoc_type)]
 #![feature(let_chains)]
 #![feature(map_try_insert)]
 #![feature(never_type)]
-#![feature(option_get_or_insert_default)]
 #![feature(round_char_boundary)]
 #![feature(try_blocks)]
 #![feature(yeet_expr)]