remove feature(nll) from #31567 test.

src/test/ui/nll/issue-31567.rs

@@ -2,8 +2,6 @@
 // causing region relations not to be enforced at all the places where
 // they have to be enforced.
 
-#![feature(nll)]
-
 struct VecWrapper<'a>(&'a mut S);
 
 struct S(Box<u32>);

src/test/ui/nll/issue-31567.stderr

@@ -1,5 +1,5 @@
 error[E0713]: borrow may still be in use when destructor runs
-  --> $DIR/issue-31567.rs:12:26
+  --> $DIR/issue-31567.rs:10:26
    |
 LL | fn get_dangling<'a>(v: VecWrapper<'a>) -> &'a u32 {
    |                 -- lifetime `'a` defined here