[DevTools] Remove invisible tab order target in network panel

Manually tabbing through the network panel with and without the search
sidebar open worked fine after removing the "tabindex = 0"

Bug: 933049
Change-Id: Ifd5c85f27c0cf0311323c5da1dd289c2fa31b27b
Reviewed-on: https://chromium-review.googlesource.com/c/1481604Reviewed-by: Joel Einbinder <einbinder@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635421}

third_party/blink/renderer/devtools/front_end/network/NetworkPanel.js

@@ -79,7 +79,6 @@ Network.NetworkPanel = class extends UI.Panel {
     const splitWidget = new UI.SplitWidget(true, false, 'networkPanelSidebarState', initialSidebarWidth);
     splitWidget.hideSidebar();
     splitWidget.enableShowModeSaving();
-    splitWidget.element.tabIndex = 0;
     splitWidget.show(this.element);
     this._sidebarLocation = UI.viewManager.createTabbedLocation(async () => {
       UI.viewManager.showView('network');