`validity()` no longer needs to return a `VortexResult` anywhere

[connortsui20]

I am pretty sure that we have no uses of validity() returning an error now.

We have this in vortex-array/src/array/vtable/validity.rs which always returns Ok:

impl<V: VTable> ValidityVTable<V> for ValidityVTableFromValidityHelper
where
    V::ArrayData: ValidityHelper,
{
    fn validity(array: ArrayView<'_, V>) -> VortexResult<Validity> {
        Ok(array.data().validity().clone())
    }
}