I have used the following CSS to remove those icons from the navigation portlet:

/* REMOVE ICONS FROM NAVTREE... */
#portal-column-one .navTreeItem a,
#portal-column-one .navTreeItem a:hover,
#portal-column-one .navTreeItem div {
    background-image: none !important;

}
/* ... AND REMOVE THE ICON PADDING */
#portal-column-one #portlet-navigation-tree .visualIconPadding {
    padding: 0;
    margin: 0;
}

You can put this CSS in the ploneCustom.css file in the portal_skins/custom folder.

(If the ploneCustom.css file is not already in the custom folder, go up to the portal_skins, click on plone_styles, and then on ploneCustom.css. Click on the "customize" button and you will automagically have a ploneCustom.css file in the custom folder and be able to immediately start editing it.)