Bug 1594123 - Fix lookup of toolbarbutton-icon in TabsList r=mconley

document.getAnonymousElementByAttribute is dead code now, and even when it wasn't this would
have returned null ever since <toolbarbutton> stopped using XBL.

Differential Revision: https://phabricator.services.mozilla.com/D51727

--HG--
extra : moz-landing-system : lando

browser/modules/TabsList.jsm

@@ -279,17 +279,7 @@ class TabsPanel extends TabsListBase {
 
   _setImageAttributes(row, tab) {
     let button = row.firstElementChild;
-    let image =
-      this.doc.getAnonymousElementByAttribute(
-        button,
-        "class",
-        "toolbarbutton-icon"
-      ) ||
-      this.doc.getAnonymousElementByAttribute(
-        button,
-        "class",
-        "toolbarbutton-icon tab-throbber-tabslist"
-      );
+    let image = button.icon;
 
     if (image) {
       let busy = tab.getAttribute("busy");