"Inbox Zero" with Apple Watch Siri watchface and Activity?
At this time, with watchOS 4.3.1, it's not possible to remove Activity from the Siri watch face.

watchOS 5 is still in beta and there's been no indication from Apple that Activity can be removed, either.  

Of course, Apple has in the past added and removed features while the software is in beta so this could change before it's officially released.