Removes oxidation states from a structure.

def remove_oxidation_states(self):
        """
        Removes oxidation states from a structure.
        """
        for site in self.sites:
            new_sp = collections.defaultdict(float)
            for el, occu in site.species.items():
                sym = el.symbol
                new_sp[Element(sym)] += occu
            site.species = new_sp