ITS: cleanup unused GPU code

[f3sch]

This cleans out ITS legacy code.
@davidrohr I am not sure about these functions:

• void RegisterPermanentMemoryAndProcessors()
• void RegisterGPUProcessors()
• void MemorySize(size_t&, size_t&)
  and left them blank as they do not seem to be needed anymore?

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[davidrohr]

Leaving them blanc is ok.