HttpClientHandler._operationStarted is never set to true

[stephentoub]

If that's ok, it should be deleted. If it's not, the appropriate set needs to be added.

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[ManickaP]

This might be remnant from Curl/WinHttpHandler removal... I'll scan the history and report back.
Nope, it wasn't used before the removals. It should get removed.

Originally moved from UAP handler code in: 247c84a#diff-108e2cc21fed31695657738cb1c81d67