Revert let_chains stabilization

This reverts commit 3266460749.

This is the revert against master, the beta revert was already done in #100538.

compiler/rustc_ast_lowering/src/lib.rs

@@ -31,6 +31,7 @@
 //! in the HIR, especially for multiple identifiers.
 
 #![feature(box_patterns)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(never_type)]
 #![recursion_limit = "256"]