Formally implement let chains

src/test/ui/expr/if/attrs/let-chains-attr.rs

@@ -1,6 +1,6 @@
 // check-pass
 
-#![feature(let_chains)] //~ WARN the feature `let_chains` is incomplete
+#![feature(let_chains)]
 
 #[cfg(FALSE)]
 fn foo() {

src/test/ui/expr/if/attrs/let-chains-attr.stderr

@@ -1,11 +0,0 @@
-warning: the feature `let_chains` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/let-chains-attr.rs:3:12
-   |
-LL | #![feature(let_chains)]
-   |            ^^^^^^^^^^
-   |
-   = note: `#[warn(incomplete_features)]` on by default
-   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
-
-warning: 1 warning emitted
-