Removed checks/workarounds for pre-C++89 compatibility (#449)

After 30+ years, just assume that the following exist:
- extension-less includes
- namespaces
- std::
- static_cast
- bool

c++/test/h5cpputil.cpp

@@ -19,11 +19,7 @@
 
  ***************************************************************************/
 
-#ifdef OLD_HEADER_FILENAME
-#include <iostream.h>
-#else
 #include <iostream>
-#endif
 using std::cerr;
 using std::endl;