Change uc_sf_order_fieldmap_objects_alter() to only add the OpportunityContactRole fields on order-related objects

This task may actually be redundant after the OpportunityContactRole node is removed, but currently it is annoying that the OpportunityContactRole integration fields show up on every type of Drupal object. I think this is because the original version of the module didn't come with a pre-defined node type for this integration. Instead you had to define it yourself and thus the module couldn't predict what the machine name was for the node type (though it could've had a configuration dropdown that set a variable for this).

The _uc_sf_order_export_values() export callback should be renamed as well, since it is only one of many export callbacks now.