Rollup merge of rust-lang#140966 - est31:let_chains_library, r=tgross35

Remove #![feature(let_chains)] from library and src/librustdoc

PR rust-lang#132833 has stabilized the `let_chains` feature. This PR removes the last occurences from the library, the compiler, and librustdoc (also because rust-lang#140887 missed the conditional in one of the crates as it was behind the "rustc" feature).

We keep `core` as exercise for the future as updating it is non-trivial (see PR thread).

Author: fmease

std/src/lib.rs

@@ -305,7 +305,6 @@
 #![feature(iter_advance_by)]
 #![feature(iter_next_chunk)]
 #![feature(lang_items)]
-#![feature(let_chains)]
 #![feature(link_cfg)]
 #![feature(linkage)]
 #![feature(macro_metavar_expr_concat)]