downloads: remove focus row background color.

The DOM is currently not set up well for a background color, making this
really ugly.

I'll get this working in the future, but it'll involve hacking up the
HTML structure a bit.

BUG=458483
R=groby@chromium.org
TEST=visual

Review URL: https://codereview.chromium.org/921273002

Cr-Commit-Position: refs/heads/master@{#316304}

chrome/browser/resources/downloads/downloads.css

@@ -29,10 +29,6 @@
   position: relative;
 }
 
-.focus-row-active {
-  background-color: rgba(0, 0, 0, .05);
-}
-
 .download,
 #no-downloads-or-results {
   margin-top: 6px;