Remove unused feature gates from librustc_metadata

Author: bjorn3

src/librustc_metadata/lib.rs

@@ -1,15 +1,11 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/")]
 #![feature(bool_to_option)]
-#![feature(box_patterns)]
 #![feature(core_intrinsics)]
 #![feature(crate_visibility_modifier)]
 #![feature(drain_filter)]
 #![feature(in_band_lifetimes)]
-#![feature(libc)]
 #![feature(nll)]
 #![feature(proc_macro_internals)]
-#![feature(proc_macro_quote)]
-#![feature(rustc_private)]
 #![feature(specialization)]
 #![feature(stmt_expr_attributes)]
 #![recursion_limit = "256"]