Remove dead CSS code from chrome://apps.

Removing not just any dead CSS code, but specifically CSS code that
relies on flattenhtml=true. This is part of the effort to remove
flattenhtml usages and auto-generate a grd file for chrome://apps.

Specifically the elements styled in the dead code (.close-button) were
 - Added at https://codereview.chromium.org/7124002 and
   https://codereview.chromium.org/7342017
 - Removed athttps://codereview.chromium.org/1129903008 and
   https://codereview.chromium.org/1753783004/ (~5 years ago).

Bug: 1152343,1169733
Change-Id: If05ffdec4d95bee6c9b146dc4a7ac86d2040f135
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2643558Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846256}

chrome/browser/resources/ntp4/new_tab.css

@@ -24,36 +24,6 @@ body {
   display: none !important;
 }
 
-.close-button {
-  background: no-repeat;
-  background-color: transparent;
-  /* TODO(estade): this should animate between states. */
-  background-image: -webkit-image-set(
-      url(../../../../ui/resources/default_100_percent/close_2.png) 1x,
-      url(../../../../ui/resources/default_200_percent/close_2.png) 2x);
-  border: 0;
-  cursor: default;
-  display: inline-block;
-  height: 16px;
-  padding: 0;
-  width: 16px;
-}
-
-.close-button:hover,
-.close-button:focus {
-  background-image: -webkit-image-set(
-      url(../../../../ui/resources/default_100_percent/close_2_hover.png) 1x,
-      url(../../../../ui/resources/default_200_percent/close_2_hover.png) 2x);
-}
-
-.close-button:active {
-  background-image: -webkit-image-set(
-      url(../../../../ui/resources/default_100_percent/close_2_pressed.png)
-          1x,
-      url(../../../../ui/resources/default_200_percent/close_2_pressed.png)
-          2x);
-}
-
 [is='action-link'] {
   margin-inline-start: 0.5em;
 }