I would simply guess that they removed it so nobody can say that Duergars will 100% sure be in final release and there is at least *some* bit of surprise in it.
OR ... and that is also quite possible ...
That they actualy didnt remove it, just forbid other races to worship him ... as far as i know (and i dont, since i never bothered to check) you also can choose Vlaakith only as Githyanki.
