net/http: remove DualStack in DefaultTransport

[ananyasaxena]

Removing DualStack from http.DefaultTransport since it is deprecated
https://github.com/golang/go/blob/master/src/net/dial.go#L61

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Patch Set 1: Code-Review+1

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Patch Set 1: Run-TryBot+1 Code-Review+2 Trust+1

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Patch Set 1: Code-Review+2 Trust+1

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This PR is being closed because golang.org/cl/253198 has been merged.