remove contacts_date_of_birth attr

When running

```
rake spree_sample:suppliers
```

I was getting a "cannot assign mass protected" error on the field :contacts_date_of_birth.

That is because this column was removed in the last migrate:

https://github.com/jdutil/spree_drop_ship/blob/master/db/migrate/20130829004248_remove_date_of_birth.rb#L3

So I removed it from the rake task