Remove obsolete `device_role` field from REST API serializer for device model

[jeremystretch]

Proposed Changes

Drop the device_role property from the Device model, and the field from DeviceSerializer. (It has been superseded by role.)

Justification

The model field was renamed to role under #6391 in v3.6; this field was retained on the serializer for backward compatibility.

Impact

Any REST API clients which still reference the device_role field must be updated to use the role field, which contains the same data (and is writable).