Stop enabling in_band_lifetimes in rustc_data_structures

There's a conversation in the tracking issue about possibly unaccepting `in_band_lifetimes`, but it's used heavily in the compiler, and thus there'd need to be a bunch of PRs like this if that were to happen.

So here's one to see how much of an impact it has.

(Oh, and I removed `nll` while I was here too, since it didn't seem needed.  Let me know if I should put that back.)

compiler/rustc_data_structures/src/lib.rs

@@ -15,13 +15,11 @@
 #![feature(core_intrinsics)]
 #![feature(extend_one)]
 #![feature(hash_raw_entry)]
-#![feature(in_band_lifetimes)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(type_alias_impl_trait)]
 #![feature(new_uninit)]
-#![feature(nll)]
 #![feature(once_cell)]
 #![feature(test)]
 #![feature(thread_id_value)]