Incorporate ProductListView into ProductCategoryView

Now that Oscar's simple search has been removed, ProductListView does
nothing but show all products. It's been incorporated into
ProductCategoryView.

ProductListView used to give out an invalid summary; this is now fixed.

ProductCategoryView used to pass 'categories' in the template context,
but it wasn't used.
Remove legacy URL route to access categories

It's been unofficially deprecated in 0.6 already.