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++/src/H5DataSet.cpp

@@ -11,11 +11,7 @@
  * help@hdfgroup.org.                                                        *
  * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
 
-#ifdef OLD_HEADER_FILENAME
-#include <iostream.h>
-#else
 #include <iostream>
-#endif
 #include <string>
 
 #include "H5private.h" // for HDfree