Drop duplicate test

First occurrence: [test/unit/test-Client.js#L219](https://github.com/felixge/node-ar-drone/blob/master/test/unit/test-Client.js#L219)
Second occurrence: [test/unit/test-Client.js#L233](https://github.com/felixge/node-ar-drone/blob/master/test/unit/test-Client.js#L233)