Drop unused string functions
Drop unused string functions

They're a bit obscure anyway. This is a port of
https://github.com/tangentlabs/django-oscar/commit/8573052b89988ecd7d68f5e0207d37934be91847

ellmetha/django-phonenumber-field,CloudNcodeInc/django-phonenumber-field,hovel/django-phonenumber-field,invalid-access/django-phonenumber-field,CloudNcodeInc/django-phonenumber-field,CloudNcodeInc/django-phonenumber-field,thenewguy/django-phonenumber-field,thenewguy/django-phonenumber-field,hwkns/django-phonenumber-field,thenewguy/django-phonenumber-field,invalid-access/django-phonenumber-field,MattBlack85/django-phonenumber-field,bramd/django-phonenumber-field,ellmetha/django-phonenumber-field,bramd/django-phonenumber-field,hwkns/django-phonenumber-field,MattBlack85/django-phonenumber-field,hovel/django-phonenumber-field,stefanfoulis/django-phonenumber-field