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_target/src/lib.rs

@@ -14,7 +14,7 @@
 #![feature(exhaustive_patterns)]
 #![feature(iter_intersperse)]
 #![feature(let_chains)]
-#![feature(min_specialization)]
+#![cfg_attr(bootstrap, feature(min_specialization))]
 #![feature(rustc_attrs)]
 #![feature(step_trait)]
 #![allow(internal_features)]