simplify access functions in distributor_service module, and use the delete permission

Issue Summary

There's no need for distributor_service_get_access() and co when all they do is run user_access(), and hook_services's '#access callback' key defaults to user_access() anyway.

Furthermore, distributor_service_delete_access() is never actually used anywhere. This means that 'delete distributed nodes' permission has no effect; presumably the intention was to have it on the node.deleteDistributedNode service.