Remove SetHttpFlavor option from httpclient instrumentations

[cijothomas]

Related to #3373

HttpClient instrumentations current have an option SetHttpFlavor which determines if 'http.flavor' is populated or not. This, I believe, was added in early days of instrumentation, when this was the only tag that was optional. Conventions have grown significantly since then, so maintaining on/off flag for each of them would be impractical.

'http.flavor' is listed as Recommended in the Spec, so it should be populated by default. SDK may provide an option to turn it off.

Given the fact that this tag's value is readily available, and is a Recommended tag, proposing to remove this On/Off option.
If any user does not want this tag, they may remove it by doing activity.SetTag("http.flavor", null) with an ActivityProcessor.

As part of overall issue tracked by #3373, we can consider exposing on/off flags in the future.