Remove all unused feature gates from the compiler

compiler/rustc_errors/src/lib.rs

@@ -15,12 +15,10 @@
 #![feature(box_patterns)]
 #![feature(default_field_values)]
 #![feature(error_reporter)]
-#![feature(if_let_guard)]
 #![feature(negative_impls)]
 #![feature(never_type)]
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
-#![feature(trait_alias)]
 #![feature(try_blocks)]
 #![feature(yeet_expr)]
 // tidy-alphabetical-end