Rollup merge of #85850 - bjorn3:less_feature_gates, r=jyn514

Remove unused feature gates

The first commit removes a usage of a feature gate, but I don't expect it to be controversial as the feature gate was only used to workaround a limitation of rust in the past. (closures never being `Clone`)

The second commit uses `#[allow_internal_unstable]` to avoid leaking the `trusted_step` feature gate usage from inside the index newtype macro. It didn't work for the `min_specialization` feature gate though.

The third commit removes (almost) all feature gates from the compiler that weren't used anyway.

compiler/rustc_query_impl/src/lib.rs

@@ -2,13 +2,9 @@
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(in_band_lifetimes)]
-#![feature(exhaustive_patterns)]
 #![feature(nll)]
 #![feature(min_specialization)]
-#![feature(crate_visibility_modifier)]
-#![feature(once_cell)]
 #![feature(rustc_attrs)]
-#![feature(never_type)]
 #![recursion_limit = "256"]
 
 #[macro_use]