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_ast/src/lib.rs

@@ -15,7 +15,7 @@
 #![feature(box_patterns)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]
-#![feature(min_specialization)]
+#![cfg_attr(bootstrap, feature(min_specialization))]
 #![feature(negative_impls)]
 #![feature(stmt_expr_attributes)]