cv: stop asking for CURRENT_REVISION when polling.
cv: stop asking for CURRENT_REVISION when polling.

It is not actually used by poller.

R=qyearsley, yiwzhang

Change-Id: Ia2bf84082a9100c85632634163290c646f72d322
Bug: 1178658
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2696147
Reviewed-by: Quinten Yearsley <747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>

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