Remove VTK custom FindOpenMP as CMake's version is now sufficient.
Now that VTK requires CMake 3.12+ we can safely use the OpenMP module provided by CMake.
In addition this fixes the following warning when trying to use the OpenMP module that is provided with VTK:
CMake Warning (dev) at /opt/cmake-3.17/share/cmake-3.17/Modules/FindPackageHandleStandardArgs.cmake:272 (message):
The package name passed to `find_package_handle_standard_args` (OpenMP_CXX)
does not match the name of the calling package (OpenMP). This can lead to
problems in calling code that expects `find_package` result variables
(e.g., `_FOUND`) to follow a certain pattern.
Call Stack (most recent call first):
CMake/FindOpenMP.cmake:487 (find_package_handle_standard_args)
CMake/vtkModule.cmake:4155 (find_package)
Common/Core/vtkSMPSelection.cmake:33 (vtk_module_find_package)
Common/Core/CMakeLists.txt:41 (include)
This warning is for project developers. Use -Wno-dev to suppress it.