Files app: Remove CSS z-index

Remove z-index from list.css because it was causing unnecessary
re-painting of breadcrumbs, toolbar buttons, and others.

Bug: 992821
Change-Id: I31e021122d74c13978bd1c1bb4603b64f8611d96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1859421
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705770}

ui/file_manager/file_manager/foreground/css/list.css

@@ -29,21 +29,6 @@ grid > * {
   display: inline-block;
 }
 
-list:focus > [lead],
-grid:focus > [lead] {
-  z-index: 2;
-}
-
-list:not([disabled]) > :hover,
-grid:not([disabled]) > :hover {
-  z-index: 1;
-}
-
-list > [selected],
-grid > [selected] {
-  z-index: 2;
-}
-
 html[class*='col-resize'] list,
 html[class*='col-resize'] grid {
   overflow-y: hidden !important;