Remove custom sorl key-value store

As far as I can tell, it was added to circumvent a problem in Sorl. The
problem was fixed in
https://github.com/mariocesar/sorl-thumbnail/pull/167, and we use a
released version that contains the fix.

I also don't see it being used anywhere. So it seems safe to delete it.

oscar/sorl_kvstore.py

@@ -1,17 +0,0 @@
-from django.core.cache import cache
-
-from sorl.thumbnail.conf import settings
-from sorl.thumbnail.kvstores.cached_db_kvstore import KVStore
-from sorl.thumbnail.models import KVStore as KVStoreModel
-
-
-class ConcurrentKVStore(KVStore):
-    """
-    Custom KV store for Sorl to avoid integrity errors when settings new
-    values.
-    """
-
-    def _set_raw(self, key, value):
-        KVStoreModel.objects.get_or_create(
-            key=key, defaults={'value': value})
-        cache.set(key, value, settings.THUMBNAIL_CACHE_TIMEOUT)