If you look in the theme.skin file of your skin you may have a settings something like this:

<portal:mojoTreeView runat="server" SkinID="ChildSiteMap"
    ContainerCssClass="AspNet-TreeView"
        
    />

you could change it like this:

<portal:mojoTreeView runat="server" SkinID="ChildSiteMap"
    ContainerElement=""
        
    />

the default value for ContainerElement is div so setting it to empty string should remove it.