Option DUNE_ENABLE_PYTHONBINDINGS not used

It seems to indicate that one would be able to deactivate python (all these env buildings), but it is never used (Only set to OFF if Python is not found. OFF is the default anyway).

I propose to remove it.

BTW what is the recommended way to skip the python (I keep getting troubles...)? -DCMAKE_DISABLE_FIND_PACKAGE_Python3=ON?