Settings WebUI: Remove double border in font subpage.

Before/after screenshots: https://imgur.com/a/0NpIjAz

Bug: None
Change-Id: I0a8db2d980ae58c489e4375e2d83505eabc424b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1524715Reviewed-by: Dan Beam <dbeam@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641058}

chrome/browser/resources/settings/appearance_page/appearance_fonts_page.html

@@ -57,7 +57,7 @@
             menu-options="[[fontOptions_]]">
         </settings-dropdown-menu>
       </div>
-      <div class="list-item underbar"
+      <div class="list-item"
           style="
               font-size:[[prefs.webkit.webprefs.default_font_size.value]]px;
               font-family:
@@ -78,7 +78,7 @@
             menu-options="[[fontOptions_]]">
         </settings-dropdown-menu>
       </div>
-      <div class="list-item underbar"
+      <div class="list-item"
           style="
               font-size:[[prefs.webkit.webprefs.default_font_size.value]]px;
               font-family:
@@ -99,7 +99,7 @@
             menu-options="[[fontOptions_]]">
         </settings-dropdown-menu>
       </div>
-      <div class="list-item underbar"
+      <div class="list-item"
           style="
               font-size:[[prefs.webkit.webprefs.default_font_size.value]]px;
               font-family: