void FieldDelete(clientContext ctx, string[] fieldNames)
{
    foreach (var field in fieldNames)
    {
        var web = ctx.Web;
        ctx.Load(web);
        ctx.ExecuteQuery();

        var field = web.Fields.GetByInternalNameOrTitle(field);
        if (field != null)
        {
            field.SetAllowDeletion(true);
            field.DeleteObject();
            web.Update();
            ctx.ExecuteQuery();
        }
    }
}