Remove custom implementations of isinf and isnan

Use the ones from the <cmath> header instead, now that that is available
on all the currently supported versions of MSVC.

glslang/Include/Common.h

@@ -292,34 +292,6 @@ template <class T> int IntLog2(T n)
     return result;
 }
 
-inline bool IsInfinity(double x) {
-#ifdef _MSC_VER
-    switch (_fpclass(x)) {
-    case _FPCLASS_NINF:
-    case _FPCLASS_PINF:
-        return true;
-    default:
-        return false;
-    }
-#else
-    return std::isinf(x);
-#endif
-}
-
-inline bool IsNan(double x) {
-#ifdef _MSC_VER
-    switch (_fpclass(x)) {
-    case _FPCLASS_SNAN:
-    case _FPCLASS_QNAN:
-        return true;
-    default:
-        return false;
-    }
-#else
-  return std::isnan(x);
-#endif
-}
-
 } // end namespace glslang
 
 #endif // _COMMON_INCLUDED_