Remove unused Product.add_category_from_breadcrumb function

It's not used anywhere in Oscar's code. It also doesn't feel right, as
adding categories via breadcrumbs is an undocumented hackish solution
anyway.
Refactor OrderSearchForm's description function

Flake8 was correctly throwing a warning that the function is too
complex. It's been split up into two functions now, and one level of
if-nesting has been removed.