remove unused file

this file was introduced in https://github.com/spree/spree_hub_connector/commit/ebda57e#diff-a38ebb92d4c05d06a12a20ee2b1db6f1R5

but its usage seems to have been removed in https://github.com/spree/spree_hub_connector/commit/3d121ac#diff-a38ebb92d4c05d06a12a20ee2b1db6f1L5