Fix a11y issue caused by missing element chromeCleanupSecondary.

https://crrev.com/c/938623 removed #chromeCleanupSecondary, but there
was an aria reference to it. This CL removes the dangling
aria-describedby.

R=ftirelo@chromium.org

Bug: 824494
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I32e69b37a911f9586e658bae2b9d4b415b7b2008
Reviewed-on: https://chromium-review.googlesource.com/974379Reviewed-by: Fabio Tirelo <ftirelo@chromium.org>
Commit-Queue: Fabio Tirelo <ftirelo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545895}

chrome/browser/resources/settings/reset_page/reset_page.html

@@ -70,8 +70,7 @@
             <div class="start">$i18n{resetCleanupComputerTrigger}</div>
             <paper-icon-button-light class="subpage-arrow">
               <button id="chromeCleanupArrow"
-                  aria-label="$i18n{resetCleanupComputerTrigger}"
-                  aria-describedby="chromeCleanupSecondary"></button>
+                  aria-label="$i18n{resetCleanupComputerTrigger}"></button>
             </paper-icon-button-light>
           </div>
         </template>