Remove the let_else feature gate from the testsuite

Result of running:

rg -l "feature.let_else" src/test/  | xargs sed -s -i "s#^...feature.let_else..\$##"

Plus manual tidy fixes.

Author: est31

src/test/ui/async-await/async-await-let-else.rs

@@ -2,7 +2,7 @@
 // revisions: drop-tracking no-drop-tracking
 // [drop-tracking] compile-flags: -Zdrop-tracking=yes
 // [no-drop-tracking] compile-flags: -Zdrop-tracking=no
-#![feature(let_else)]
+
 use std::rc::Rc;
 
 async fn foo(x: Option<bool>) {

src/test/ui/let-else/issue-100103.rs

@@ -2,7 +2,7 @@
 // check-pass
 
 #![feature(try_blocks)]
-#![feature(let_else)]
+
 
 fn main() {
     let _: Result<i32, i32> = try {

src/test/ui/let-else/issue-94176.rs

@@ -1,6 +1,6 @@
 // Issue #94176: wrong span for the error message of a mismatched type error,
 // if the function uses a `let else` construct.
-#![feature(let_else)]
+
 
 pub fn test(a: Option<u32>) -> Option<u32> { //~ ERROR mismatched types
     let Some(_) = a else { return None; };

src/test/ui/let-else/issue-99975.rs

@@ -1,7 +1,7 @@
 // run-pass
 // compile-flags: -C opt-level=3 -Zvalidate-mir
 
-#![feature(let_else)]
+
 
 fn return_result() -> Option<String> {
     Some("ok".to_string())

src/test/ui/let-else/let-else-allow-in-expr.rs

@@ -1,5 +1,3 @@
-#![feature(let_else)]
-
 #![deny(unused_variables)]
 
 fn main() {

src/test/ui/let-else/let-else-allow-in-expr.stderr

@@ -1,17 +1,17 @@
 error: unused variable: `x`
-  --> $DIR/let-else-allow-in-expr.rs:7:13
+  --> $DIR/let-else-allow-in-expr.rs:5:13
    |
 LL |         let x = 1;
    |             ^ help: if this is intentional, prefix it with an underscore: `_x`
    |
 note: the lint level is defined here
-  --> $DIR/let-else-allow-in-expr.rs:3:9
+  --> $DIR/let-else-allow-in-expr.rs:1:9
    |
 LL | #![deny(unused_variables)]
    |         ^^^^^^^^^^^^^^^^
 
 error: unused variable: `x`
-  --> $DIR/let-else-allow-in-expr.rs:29:9
+  --> $DIR/let-else-allow-in-expr.rs:27:9
    |
 LL |     let x = 1;
    |         ^ help: if this is intentional, prefix it with an underscore: `_x`

src/test/ui/let-else/let-else-allow-unused.rs

@@ -1,6 +1,6 @@
 // issue #89807
 
-#![feature(let_else)]
+
 
 #[deny(unused_variables)]
 

src/test/ui/let-else/let-else-binding-explicit-mut-annotated.rs

@@ -1,6 +1,6 @@
 // from rfc2005 test suite
 
-#![feature(let_else)]
+
 
 // Verify the binding mode shifts - only when no `&` are auto-dereferenced is the
 // final default binding mode mutable.

src/test/ui/let-else/let-else-binding-explicit-mut-borrow.rs

@@ -1,8 +1,8 @@
-#![feature(let_else)]
-
 // Slightly different from explicit-mut-annotated -- this won't show an error until borrowck.
 // Should it show a type error instead?
 
+
+
 fn main() {
     let Some(n): &mut Option<i32> = &mut &Some(5i32) else {
         //~^ ERROR cannot borrow data in a `&` reference as mutable

src/test/ui/let-else/let-else-binding-explicit-mut-pass.rs

@@ -1,6 +1,6 @@
 // check-pass
 
-#![feature(let_else)]
+
 
 fn main() {
     let Some(n) = &mut &mut Some(5i32) else { return; };