Remove workaround for old FindHDF5 problems
The workaround that is being removed in this MR causes problems that VTK libraries can link to both, the debug and the release library of third party libraries.
Since the comment says the workaround is unnecessary with CMake >= 2.8.12.2 and the minimum required CMake version to build VTK nowadays is 3.3 it should be save to just remove the workaround completely.