Remove Order payments association's inverse_of

This was causing spree/spree_gateway#132, and would've caused similar issues for all non-token gateways (i.e. gateways that send CC info rather than getting a token and using that instead)

Super incredibly glad that we found this out before master was released as a real gem.
Use pure delegate in Spree::Gateway, compared with delegate_belongs_to

The provider method here is not a belongs_to association, so there's no good reason why we should be using delegate_belongs_to at all. Further more, delegate_belongs_to defines setters for these methods, but that is not necessary at all since these methods do not have setters!