Removed ThirdParty-folder

• ParaView class vtkCleanUnstructuredGrid is now in VTK 9.3 (bumps minimum VTK to 9.3)
• Removed ogs6py submodule
• Adapted ogs6py-dependent ctests

TODOs:

• Update VTK in Guix, https://issues.guix.gnu.org/70323
• Update guix channels

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1. Feature description was added to the changelog
2. Tests covering your feature were added?