requestIdleCallback: remove hard dependency for now

js/Headlines.js

@@ -40,9 +40,12 @@ define(["dojo/_base/declare"], function (declare) {
 			Headlines.updateSelectedPrompt();
 			Headlines.updateFloatingTitle(true);
 
-			window.requestIdleCallback(() => {
+			if ('requestIdleCallback' in window)
+				window.requestIdleCallback(() => {
+					Headlines.syncModified(modified);
+				});
+			else
 				Headlines.syncModified(modified);
-			});
 		}),
 		syncModified: function(modified) {
 			const ops = {

js/tt-rss.js

@@ -95,7 +95,7 @@ require(["dojo/_base/kernel",
 				checkBrowserFeatures: function() {
 					let errorMsg = "";
 
-					['requestIdleCallback', 'MutationObserver'].each(function(wf) {
+					['MutationObserver'].each(function(wf) {
 						if (! (wf in window)) {
 							errorMsg = `Browser feature check failed: <code>window.${wf}</code> not found.`;
 							throw $break;