utils: result_loop: remove invalid and incorrect constraint

Checking a concept in a requires-expression requires an additional
requires keyword. Moreover, the constraint is incorrect (at least
all callers pass a T, not a result<T>), so remove it.

Found by gcc 12.

utils/result_loop.hh

@@ -229,7 +229,6 @@ public:
 
 template<typename Iterator, typename Mapper, typename Initial, typename Reducer>
 requires requires (Iterator i, Mapper mapper, Initial initial, Reducer reduce) {
-    ExceptionContainerResult<Initial>;
     *i++;
     { i != i } -> std::convertible_to<bool>;
     { mapper(*i) } -> ExceptionContainerResultFuture<>;