Drop setting for custom Sorl KV store

When I removed the custom KV store in
8d73c0231428acf16da3f5a187154ea73d5fe7ef, I mistakenly only searched for
usages within the oscar/ directory, and missed that it's still specified
in the sandbox settings.

This is now fixed. Was reported on the mailing list:
https://groups.google.com/d/msgid/django-oscar/042d2f5d-e14e-4431-9dbc-286d3215b3da%40googlegroups.com?utm_medium=email&utm_source=footer