Remove unnecessary Python 2 conflict in ParaView's Spack file

@utkarsh.ayachit @patchett2002 @pflarr

In ParaView's Spack package file, there is a conflict preventing users from building newer ParaView versions with Python 2. This should be removed, because it is an unnecessary conflict and a boundary preventing LANL code teams from moving to newer ParaView versions.

https://github.com/spack/spack/blob/2421d903b034c92660fbe999b7d486e6f21a7417/var/spack/repos/builtin/packages/paraview/package.py#L54

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