Remove an attempted error annotation

This file previously contained an error annotation with incorrect syntax.
Remove this annotation, as per the dev guide, since this is marked as known-bug.
https://rustc-dev-guide.rust-lang.org/tests/ui.html#known-bugs

Author: theemathas

tests/ui/traits/const-traits/const_closure-const_trait_impl-ice-113381.rs

@@ -13,6 +13,4 @@ impl Foo for () {
 
 fn main() {
     (const || (()).foo())();
-    // ^ ERROR: cannot call non-const method `<() as Foo>::foo` in constant functions
-    // FIXME(const_trait_impl) this should probably say constant closures
 }