remove unused #![feature(async_closure)]

Co-Authored-By: Mazdak Farrokhzad <[email protected]>

Author: delan

src/test/ui/async-await/async-unsafe-fn-call-in-safe.rs

@@ -1,6 +1,6 @@
 // edition:2018
 
-#![feature(async_await, async_closure)]
+#![feature(async_await)]
 
 struct S;