Revert let_chains stabilization

This reverts commit 326646074940222d602f3683d0559088690830f4.

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

Author: Noratrieb

std/src/lib.rs

@@ -254,6 +254,7 @@
 #![feature(intra_doc_pointers)]
 #![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(lang_items)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(linkage)]
 #![feature(link_cfg)]