You need to target the bookmark's icon. Try the following:
@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul); .bookmark-item .toolbarbutton-icon[label="Read Later"] { display: none !important; }
Alternatively, you can target all blank bookmark icons (that are not in folders) with the following:
@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul); .bookmark-item .toolbarbutton-icon:not([type="menu"]):not([src]) { display: none !important; }