Settings WebUI: Remove unnecessary z-index

This layers the Settings page's main contents above the toolbar such
that the main content is interactable no matter how far the toolbar's
text overlaps the main content.

Bug: 995686
Change-Id: Id8e244a47a13f19c13b47ce86e8b44afc6255524
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762062Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688665}

chrome/browser/resources/settings/settings_ui/settings_ui.html

@@ -43,7 +43,6 @@
       cr-toolbar {
         @apply --layout-center;
         min-height: 56px;
-        z-index: 2;
         --cr-toolbar-left-spacer-width: var(--settings-menu-width);
         --cr-toolbar-center-basis: var(--settings-main-basis);
       }