Finish bumping stage0

It looks like the last time had left some remaining cfg's -- which made me think
that the stage0 bump was actually successful. This brings us to a released 1.62
beta though.

compiler/rustc_session/src/lib.rs

@@ -1,6 +1,5 @@
 #![feature(if_let_guard)]
 #![feature(let_chains)]
-#![cfg_attr(bootstrap, feature(derive_default_enum))]
 #![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(never_type)]