maths: remove util::is_nan in preference of std::isnan

cast.hpp

@@ -113,7 +113,7 @@ namespace util::cast {
         auto dst = static_cast<DstT> (src);
 
         if constexpr (std::is_floating_point_v<SrcT>) {
-            if (util::is_nan (src)) {
+            if (std::isnan (src)) {
                 // NaNs must remaing as NaN's. They're important.
                 CHECK (std::is_floating_point_v<DstT>);
                 CHECK (std::isnan (dst));

maths.hpp

@@ -188,30 +188,6 @@ namespace util {
     }
 
 
-    //-------------------------------------------------------------------------
-    template <typename T>
-    constexpr
-    typename std::enable_if_t<
-        std::is_floating_point<T>::value, bool
-    >
-    is_nan (T t)
-    {
-        return std::isnan (t);
-    }
-
-
-    //-------------------------------------------------------------------------
-    template <typename T>
-    constexpr
-    typename std::enable_if_t<
-        !std::is_floating_point<T>::value, bool
-    >
-    is_nan (const T&)
-    {
-        return false;
-    }
-
-
     ///////////////////////////////////////////////////////////////////////////
     // exponentials