Fix Nub type family

[3noch]

I don't think it works on non-consecutive duplicates.

[fizruk]

Here's efficient and correct Nub implementation (from servant-swagger):

-- | Remove duplicates from a type-level list.
type family Nub xs where
  Nub '[] = '[]
  Nub (x ': xs) = x ': Nub (Remove x xs)

-- | Remove element from a type-level list.
type family Remove x xs where
  Remove x '[]       = '[]
  Remove x (x ': ys) =      Remove x ys
  Remove x (y ': ys) = y ': Remove x ys