[gce] document config package remove unused public export.
[gce] document config package remove unused public export.

R=smut

Change-Id: Iadb6c040c350ce2946bc54560f85e4b922864581
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2337893
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: smut <95deb7aa15d4c615299e47e7688551456568b1fb@google.com>

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