Removes #line_item from 2Checkout helper

Ping @bslobodin 

This change is due to Shopify’s usage of `#line_item` conflict with the one that 2Checkout provides in its helper – same method name but different expectations of the passed params.

We could likely refactor this helper to continue working, but as we’re not using it, we’re going to err on the side that no-one is using it and just remove it for now.