remove let_chain feature

Summary:
X-link: meta-pytorch/monarch#1388

let_chains were stabilized in 1.88.0 for edition 2024 and removed in 1.90.0
rust-lang/rust#132833
rust-lang/rust#143214

Test Plan: ci

Reviewed By: dtolnay

Differential Revision: D83221093

fbshipit-source-id: 6fb40aa7e1c282f453394db5b7f0dab1282c2e35

Author: cjlongoria

antlir/antlir2/features/install/install.rs

@@ -5,7 +5,6 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-#![feature(let_chains)]
 use std::collections::HashMap;
 use std::fs::File;
 use std::fs::FileTimes;

antlir/antlir2/features/tarball/tarball.rs

@@ -5,8 +5,6 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-#![feature(let_chains)]
-
 use std::fs::File;
 use std::io::BufReader;
 use std::os::unix::ffi::OsStrExt;