Revert "libcom: properly declare isnan() in C++ code"

This reverts commit 2e89a60c2d.

src/libCom/cppStd/epicsAlgorithm.h

@@ -13,9 +13,7 @@
 #ifndef __EPICS_ALGORITHM_H__
 #define __EPICS_ALGORITHM_H__
 
-#include <cmath>
-
-using std::isnan;
+#include "epicsMath.h"
 
 // The C++ standard only requires types to be less-than comparable, so
 // the epicsMin and epicsMax templates only use operator <