WebUI Dark Mode: remove inapplicable CSS

As far as I can tell, this CSS doesn't work. Removing it instead of
attempting to update it for dark mode.

R=dpapad@chromium.org
BUG=926658

Change-Id: I34213a54828c2becc3b59cfcdd60f8de72bdad2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506389Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Dan Beam (slow; perfing) <dbeam@chromium.org>
Auto-Submit: Dan Beam (slow; perfing) <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638421}

chrome/browser/resources/settings/site_settings/category_setting_exceptions.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 
-<link rel="import" href="../settings_shared_css.html">
 <link rel="import" href="constants.html">
 <link rel="import" href="site_list.html">
 
 <dom-module id="category-setting-exceptions">
   <template>
-    <style include="settings-shared">
-      site-list {
-        border-bottom: 1px solid var(--paper-grey-300);
-      }
-
-      site-list:last-of-type {
-        border-bottom: none;
-      }
-    </style>
     <site-list
         category="[[category]]"
         category-subtype="[[ContentSetting.BLOCK]]"