client/cmd/rpc: fix -verbose flag
client/cmd/rpc: fix -verbose flag

logging framework has its own level filtering. Use it instead of setting
level at lower level.

R=dnj@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1685083004

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