Patient.save called twice during update

[mbarton]

national-paediatric-diabetes-audit/project/npda/views/patient.py

Line 322 in 65fd924

return super().form_valid(form)

super().form_valid(form) will also call patient.save.

I noticed this because I saw two calls to imd_for_postcode. After #325 we calculate imd in PatientForm.save so this is less of an issue - fixing it would just save a database call.