Settings WebUI: remove horizontal bar in security keys subpage.

Based on other subpages, the first row should not have a horizontal line
preceding it.

Bug: 947929
Change-Id: I520e78147f3e2e84079993dfabe5041d7bbd6d0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1548237
Commit-Queue: Adam Langley <agl@chromium.org>
Auto-Submit: Adam Langley <agl@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646637}

chrome/browser/resources/settings/privacy_page/security_keys_subpage.html

@@ -14,7 +14,6 @@
 
     <cr-link-row
       id="setPINButton"
-      class="hr"
       label="$i18n{securityKeysSetPIN}"
       sub-label="$i18n{securityKeysSetPINDesc}"
       on-click="onSetPIN_"></cr-link-row>