File manager CSS: remove an unneeded files-ng .file-row border

The files-ng directorytree CSS uses box-sizing: border-box, and does
not need a .file-row transparent border anymore. Remove it.

Bug: 992819
Change-Id: Ie007ef3d8783c53c7ae8c636404f87c4bddd824c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1943774Reviewed-by: Alex Danilo <adanilo@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Auto-Submit: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720363}

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

@@ -155,7 +155,6 @@ html[class*='col-resize'] #directory-tree {
   align-items: center;
   border-inline-start-width: 0;
   border-radius: 0 20px 20px 0;
-  border: 1px solid transparent;
   box-sizing: border-box;
   color: var(--google-grey-700);
   display: flex;