Remove LangItems::require

It's just a short wrapper used by `tcx.require_lang_item`. Deleting it
gives us a negative diff.

compiler/rustc_hir/src/lib.rs

@@ -30,7 +30,6 @@ pub mod def;
 pub mod def_path_hash_map;
 pub mod definitions;
 pub mod diagnostic_items;
-pub mod errors;
 pub use rustc_span::def_id;
 mod hir;
 pub mod hir_id;