Remove symmetry group workaround once AiiDA 2.6 is supported throughout

[edan-bainglass]

In #1055, I had to implement a workaround in obtaining the structure/molecule symmetry group. This is due to a rigid check on PBC that only provided pymatgen Structure objects for 3D systems, Molecule otherwise. This has since been removed and is accurate in AiiDA 2.6. Once we update throughout, we can simply use orm.StructureData.get_pymatgen() to extract the correct pymatgen object.