change command "DELETE" to some other name like "AdsDelete" and change your condition as well
else if (e.CommandName == "AdsDelete")
{
Or you can add the event
OnRowDeleting="gv_ads_RowDeleting"
to your grid like below
<asp:gridview id="gv_ads" runat="server" onrowdeleting="gv_ads_RowDeleting" xmlns:asp="#unknown"></asp:gridview>
generate the event code or write the event as below
public void gv_ads_RowDeleting(Object sender, GridViewDeleteEventArgs e)
{
}