[resultdb] Remove `-resultdb-prod` flag, it is unused.
[resultdb] Remove `-resultdb-prod` flag, it is unused.

R=chanli@chromium.org, ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org, 1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org

Change-Id: Ibc2d38009252df84733b2207cc42c0e3be36d9f2
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2351173
Reviewed-by: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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