[flat.set.modifiers] Drop `return` statement from code block in *Effects* element

[JohelEGP]

template<class K> pair<iterator, bool> insert(K&& x);
template<class K> iterator insert(const_iterator hint, K&& x);

Part of the effects of these overloads specify:

inserts a new element as if by: return emplace(std::forward<K>(x));.

These are the problems with it, first described at #5696 (comment):

1. The Returns element fully describes the result.
2. The use of return has nothing to do with how the new element is inserted.
3. And also, this return can't work with the first overload.