Removed unnecessary #ifdef __cplusplus from .cpp sources

Code review feedback:

The following files:
src/jdk.jpackage/windows/native/libjpackage/WindowsRegistry.cpp,
src/jdk.jpackage/windows/native/libjpackage/jpackage.cpp, and
src/jdk.jpackage/windows/native/libwixhelper/libwixhelper.cpp
are completely wrapped with:

#ifdef __cplusplus
extern "C" {
#endif
...
#ifdef __cplusplus
}
#endif

Would it be better to remove these wrappers and either write the source in C++ or move it to ".c" files ?