xo-unit: restore constexpr quantity*quantity

include/xo/unit/quantity_ops.hpp

@@ -6,20 +6,9 @@
 #pragma once
 
 #include "quantity_concept.hpp"
-//#include <cstdint>
 
 namespace xo {
     namespace qty {
-        /** note: won't have constexpr result until c++26 (when ::sqrt(), ::pow() are constexpr)
-         **/
-        template <typename Quantity, typename Quantity2>
-        requires quantity_concept<Quantity> && quantity_concept<Quantity2>
-        constexpr auto
-        operator* (const Quantity & x, const Quantity2 & y)
-        {
-            return Quantity::multiply(x, y);
-        }
-
         /** note: does not require unit scaling,  so constexpr with c++23 **/
         template <typename Dimensionless, typename Quantity>
         requires std::is_arithmetic_v<Dimensionless> && quantity_concept<Quantity>