luci_deploy: Remove "preview" tag.
luci_deploy: Remove "preview" tag.

Looks like "gcloud" has graduated the "app deploy" command. Remove the
"preview" tag.

TBR=iannucci@chromium.org
BUG=None
TEST=local
  - Ran without tag, deployment was successful.

Review-Url: https://codereview.chromium.org/2700463002

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