Revert "Properly declare isnan() in C++ testcode"

This reverts commit 07b75e4543.

std:: qualified names not portable until c++11
epicsMath.h changed to ensure that unqualified
names continue to work.

testApp/misc/testTypeCast.cpp

@@ -7,7 +7,6 @@
 #include <fstream>
 #include <iostream>
 #include <algorithm>
-#include <cmath>
 #include <limits>
 #include <typeinfo>
 #include <stddef.h>
@@ -24,7 +23,6 @@
 #include "pv/typeCast.h"
 
 using std::string;
-using std::isnan;
 
 namespace {
     // Mangle value to be printable as per print_convolute<>