Vapor barriers are a very confusing subject. Go ahead and install the Kerdi membrane and leave the barrier on the insulation as is. You do not want plastic on the studs behind wall board or concrete board and a surface membrane too. One or the other, not both. But.....the surface membrane is much better cuz it keeps the substrate dry.
I guess the question could also be asked like this: if tile could be applied directly on top of a clear plastic vapor barrier, would these vapor barriers all be installed on top of the concrete board instead of behind it?
So, yes. That's why you've chosen Kerdi, consider it a plastic you can tile over.