Remove dependency on rmf_dispenser_msgs

[Yadunund]

The rmf_task::requests::Delivery request depends on rmf_dispenser_msgs to store the list of items to dispense. This was implemented purely out of convenience for creating a rmf_fleet_adapter::Task in downstream rmf_fleet_adapter.
But since the introduction of changes in open-rmf/rmf_ros2#81 , this is no longer required. Removing this dependency will remove any ROS 2 dependency from this repo which should have been the case from the start