Revert "[NTP] Clear attribution on the existing tab when apply a theme"

This reverts commit a9ed1fd3.

Reason for revert: Speculative revert since two WebRTC bots started failing reliably after this CL.
If the revert does not fix the bots (which I think is likely), we will reland it.

Original change's description:
> [NTP] Clear attribution on the existing tab when apply a theme
> 
> Now we will clear the attribution also on the existing tab when a theme
> is applied.
> 
> Bug: 875519
> Change-Id: I65e8a0dc1ed3492c512cabcf1a2d3f0e6f2e64f7
> Reviewed-on: https://chromium-review.googlesource.com/1181684
> Commit-Queue: Weilun Shi <sweilun@chromium.org>
> Reviewed-by: Kristi Park <kristipark@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#584964}

TBR=kristipark@chromium.org,ramyan@chromium.org,kmilka@chromium.org,sweilun@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 875519
Change-Id: I20ff372903d2abee0620d42e9cff18777515ac65
Reviewed-on: https://chromium-review.googlesource.com/1186762Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585499}

chrome/browser/resources/local_ntp/local_ntp.js

@@ -372,7 +372,6 @@ function renderTheme() {
     window.setTimeout(function() {
       $('custom-bg').style.backgroundImage = '';
     }, 1000);
-    customBackgrounds.clearAttribution();
   }
 
   $(customBackgrounds.IDS.RESTORE_DEFAULT)

chrome/test/data/local_ntp/local_ntp_browsertest.html

@@ -71,15 +71,6 @@
       <div id="edit-bg" hidden>
         <button id="edit-bg-gear"></button>
       </div>
-
-      <div id="message-box-container" class="message-box-hide">
-        <div id="message-box">
-          <div id="message-box-icon"></div>
-          <div id="message-box-message"></div>
-          <div id="message-box-link" class="ripple"></div>
-        </div>
-      </div>
-      <div id="custom-bg-attr"></div>
     </div>
 
     <dialog div id="edit-bg-dialog">