Remove preds::unconstrained

I plan to add some built-in predicates, but not yet. This one
in particular was a hacky from when I was experimenting with
conversions.

Author: rvarago

include/rvarago/refined.hpp

@@ -68,15 +68,6 @@ struct to_exception {
 
 } // namespace error
 
-// Reusable predicates.
-namespace preds {
-
-// Assumes all values of `T` are valid.
-template <typename T> constexpr auto unconstrained(T const &) -> bool {
-  return true;
-}
-} // namespace preds
-
 template <typename T, std::predicate<T const &> auto Pred, typename... Bases>
   requires((std::is_same_v<T, typename Bases::value_type> && ...) &&
            (std::predicate<typename Bases::predicate_type, T const &> && ...))