Remove old tracing code now that it has moved to third_party.

chrome/browser/resources/tracing/overlay.css

-/* Copyright (c) 2012 The Chromium Authors. All rights reserved.
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file. */
-
-.overlay-root {
-  -webkit-box-align: stretch;
-  -webkit-box-orient: horizontal;
-  -webkit-box-pack: center;
-  -webkit-user-select: none;
-  background: rgba(0, 0, 0, 0.8);
-  display: -webkit-box;
-  height: 100%;
-  left: 0;
-  position: fixed;
-  top: 0;
-  width: 100%;
-  z-index: 1000;
-}
-
-.overlay-root:not([visible]),
-.overlay:not([visible]) {
-  display: none;
-}
-
-.overlay-root > .content-host {
-  -webkit-box-align: stretch;
-  -webkit-box-orient: vertical;
-  -webkit-box-pack: center;
-  -webkit-user-select: auto;
-  display: -webkit-box;
-}
-
-.overlay-root > .content-host > * {
-  background: rgb(255, 255, 255);
-}