Remove default xsd and xsi namespaces from MVC xml response

I need to remove the xsd and xsi namespaces from my MVC api response but I can't make it work. (The reason I need to remove them is because we're sending it to a third party map provider, and it doesn't work if you include them.) I've read a bunch of other questions, for example this which is asking the exact same thing but the answer doesn't work, and I've tried to implement a custom formatter as per this answer, but no luck there either unfortunately. Does anyone know what I need to do? Should I just give up and use a handler instead?

Model:

[XmlType(TypeName = "MapPoints")]
[XmlRoot(DataType = "", Namespace = "http://tempuri.org/XMLFile1.xsd")]
public class MapPointList
{
    public MapPointList()
    {
        Points = new List<MapPoint>();
    }

    [XmlElement("Point")]
    public List<MapPoint> Points { get; set; }
}

Controller:

    public class MyController : ApiController
{
    public MyController()
    {
        var xmlFormatter = GlobalConfiguration.Configuration.Formatters.XmlFormatter;
        xmlFormatter.SetSerializer<MapPointList>(new XmlSerializer(typeof (MapPointList)));
        xmlFormatter.SetSerializer<MapPoint>(new XmlSerializer(typeof (MapPoint)));
    }

    [HttpGet]
    public MapPointList SearchMap()
    {
        var items = new MapPointList();
        // get the items

        return items;
    }
}

My root should look like this:

<MapPoints xmlns="http://tempuri.org/XMLFile1.xsd">

Let me know if I need to add any more information.