Delete Read only field from Sharepoint list/library

How to remove/delete a read only field programatically? 

SPSecurity.RunWithElevatedPrivileges(delegate

{

    using (SPSite spSite = new SPSite("siteurl"))

        using (SPWeb spWeb = spSite.OpenWeb())

        {

          SPList list = spWeb.Lists["ListName"];

                    SPField spField = list.Fields["ColumnName"];
                    spField.ReadOnlyField = false ;
                    spField.Update();
                    spField.Delete();
                    spField.ParentList.Update();

        }

});