CSharpCompletionCommandHandlerTests.TestDeletingWholeWordResetCompletionToTheDefaultItem is flakey

[CyrusNajmabadi]

I've had unit tests fail at least a dozen times in this test. Disabling.

https://dev.azure.com/dnceng-public/public/_build/results?buildId=544838&view=ms.vss-test-web.build-test-results-tab&runId=12933342&resultId=223412&paneView=debug