DevTools: remove old Timeline front-end implementation

LayoutTests/http/tests/inspector/inspector-test.js

 var initialize_InspectorTest = function() {
 
 var results = [];
 
 function consoleOutputHook(messageType)
 {
     InspectorTest.addResult(messageType + ": " + Array.prototype.slice.call(arguments, 1));
 }
 
 console.log = consoleOutputHook.bind(InspectorTest, "log");
@@ skipping 677 matching lines @@
         var lastLoadedPanel;
 
         var promises = [];
         for (var i = 0; i < InspectorTest._panelsToPreload.length; ++i) {
             lastLoadedPanel = InspectorTest._panelsToPreload[i];
             promises.push(WebInspector.inspectorView.panel(lastLoadedPanel));
         }
 
         var testPath = WebInspector.settings.testPath.get();
 
-        // FIXME(399531): enable timelineOnTraceEvents experiment when running layout tests under inspector/tracing/. This code
-        // should be removed along with the old Timeline implementation once we move tracing based Timeline out of experimental.
-        if (testPath.indexOf("tracing/") !== -1)
-            Runtime.experiments.setEnabled("timelineOnTraceEvents", true);
-
         if (testPath.indexOf("layers/") !== -1)
             Runtime.experiments.setEnabled("layersPanel", true);
 
         // 2. Show initial panel based on test path.
         var initialPanelByFolder = {
             "audits": "audits",
             "console": "console",
             "elements": "elements",
             "editor": "sources",
             "layers": "layers",
@@ skipping 97 matching lines @@
     _output("[page] " + text);
 }
 
 function _output(result)
 {
     if (!outputElement)
         createOutputElement();
     outputElement.appendChild(document.createTextNode(result));
     outputElement.appendChild(document.createElement("br"));
 }