[swarming] remove conccurentHTTP2 RoundTripper
[swarming] remove conccurentHTTP2 RoundTripper

This is revert of
https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1343912

We don't need to have this hack in go1.12.
See detail in
https://golang.org/doc/go1.12#net/http

Bug: 868878
Change-Id: I5a23eace6eefca2be1ee6d645a6aa31308b60bb3
Reviewed-on: https://chromium-review.googlesource.com/c/1488591
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go