Revert let_chains stabilization

This reverts commit 3266460.

It was discovered that they are not implemented correctly, which
does not make them ready for stabilization.

Author: Noratrieb

compiler/rustc_ast/src/lib.rs

@@ -14,7 +14,7 @@
 #![feature(const_trait_impl)]
 #![feature(if_let_guard)]
 #![feature(label_break_value)]
-#![cfg_attr(bootstrap, feature(let_chains))]
+#![feature(let_chains)]
 #![feature(min_specialization)]
 #![feature(negative_impls)]
 #![feature(slice_internals)]

compiler/rustc_ast_lowering/src/lib.rs

@@ -31,7 +31,7 @@
 //! in the HIR, especially for multiple identifiers.
 
 #![feature(box_patterns)]
-#![cfg_attr(bootstrap, feature(let_chains))]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(never_type)]
 #![recursion_limit = "256"]