[milo] log if someone uses GetBuildbotBuildsJSON with cursor
[milo] log if someone uses GetBuildbotBuildsJSON with cursor

According to my code search results, nobody uses GetBuildbotBuildsJSON
API with a cursor. Log to confirm.

R=40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org, hinoka@chromium.org

Bug:
Change-Id: Icba994800ae1debb67b5fc775c1b7d03468dd47b
Reviewed-on: https://chromium-review.googlesource.com/680257
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

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