swarming: log the number of skipped bot in cron_delete_old_bot
swarming: log the number of skipped bot in cron_delete_old_bot

Bug: 1117402
Change-Id: Ie6618cbbe7425c23e45d181be0f644b070d7e53f
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2691767
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>
Reviewed-by: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py