Remove let_chains feature gate from some places in the testsuite

Author: est31

tests/ui/expr/if/attrs/let-chains-attr.rs

@@ -1,6 +1,6 @@
 //@ check-pass
-
-#![feature(let_chains)]
+//@ edition:2024
+//@ compile-flags: -Zunstable-options
 
 #[cfg(FALSE)]
 fn foo() {

tests/ui/unpretty/expanded-exhaustive.rs

@@ -13,7 +13,6 @@
 #![feature(deref_patterns)]
 #![feature(explicit_tail_calls)]
 #![feature(gen_blocks)]
-#![feature(let_chains)]
 #![feature(more_qualified_paths)]
 #![feature(never_patterns)]
 #![feature(never_type)]

tests/ui/unpretty/expanded-exhaustive.stdout

@@ -14,7 +14,6 @@
 #![feature(deref_patterns)]
 #![feature(explicit_tail_calls)]
 #![feature(gen_blocks)]
-#![feature(let_chains)]
 #![feature(more_qualified_paths)]
 #![feature(never_patterns)]
 #![feature(never_type)]

tests/ui/unpretty/expanded-interpolation.rs

@@ -1,4 +1,5 @@
-//@ compile-flags: -Zunpretty=expanded
+//@ compile-flags: -Zunpretty=expanded -Zunstable-options
+//@ edition:2024
 //@ check-pass
 
 // This test covers the AST pretty-printer's insertion of parentheses in some
@@ -7,7 +8,6 @@
 // Rust syntax. We also test negative cases: the pretty-printer should not be
 // synthesizing parentheses indiscriminately; only where necessary.
 
-#![feature(let_chains)]
 #![feature(if_let_guard)]
 
 macro_rules! expr {

tests/ui/unpretty/expanded-interpolation.stdout

@@ -1,6 +1,6 @@
 #![feature(prelude_import)]
-#![no_std]
-//@ compile-flags: -Zunpretty=expanded
+//@ compile-flags: -Zunpretty=expanded -Zunstable-options
+//@ edition:2024
 //@ check-pass
 
 // This test covers the AST pretty-printer's insertion of parentheses in some
@@ -9,10 +9,9 @@
 // Rust syntax. We also test negative cases: the pretty-printer should not be
 // synthesizing parentheses indiscriminately; only where necessary.
 
-#![feature(let_chains)]
 #![feature(if_let_guard)]
 #[prelude_import]
-use ::std::prelude::rust_2015::*;
+use std::prelude::rust_2024::*;
 #[macro_use]
 extern crate std;