[Ailae]

They removed the Interface-option and its associated cVar as far as I can tell, but you can hide it with this:

local f = CreateFrame("Frame", nil, UIParent)
f:RegisterEvent("ADDON_LOADED")
f:SetScript("OnEvent", function(self, event, name)
    if name == "Blizzard_TimeManager" then
        TimeManagerClockButton:Hide()
        TimeManagerClockButton:SetScript("OnShow", function(self)
            TimeManagerClockButton:Hide()
        end)
    end
end)

You have to register for ADDON_LOADED since TimeManagerClockButton doesn't exist until Blizzard_TimeManager is loaded.