oscar1.0: Remove PaymentDetailsView.handle_place_order_submission

PaymentDetailsView.handle_place_order_submission method provided by the
cashondelivery app inserts billing_address into the submission data
returned by the build_submission method.

In oscar 1.0, the submission data returned by the build_submission method
already has a billing_address key. Refer:
https://django-oscar.readthedocs.org/en/releases-1.0/releases/v1.0.html#billing-addresses-explicitly-passed-around-checkout

The multiple billing_addresses being passed to the submit method
results in an error while trying to invoke the handle_order_placement method.
This commit removes the now obselete handle_place_order_submission method to prevent this error.