Remove unnecessary min_specialization after bootstrap

These crates all needed specialization for `newtype_index!`, which will no
longer be necessary when the current nightly eventually becomes the next
bootstrap compiler.

compiler/rustc_mir_transform/src/lib.rs

@@ -8,7 +8,7 @@
 #![feature(is_sorted)]
 #![feature(let_chains)]
 #![feature(map_try_insert)]
-#![feature(min_specialization)]
+#![cfg_attr(bootstrap, feature(min_specialization))]
 #![feature(never_type)]
 #![feature(option_get_or_insert_default)]
 #![feature(round_char_boundary)]