Remove unnecessary feature gates

One wasn't necessary anymore at all. Others are only necessary when jit mode is enabled.

Author: bjorn3

src/lib.rs

@@ -1,4 +1,5 @@
-#![feature(rustc_private, decl_macro, never_type, hash_drain_filter, vec_into_raw_parts, once_cell)]
+#![feature(rustc_private, decl_macro)]
+#![cfg_attr(feature = "jit", feature(never_type, vec_into_raw_parts, once_cell))]
 #![warn(rust_2018_idioms)]
 #![warn(unused_lifetimes)]
 #![warn(unreachable_pub)]