Remove /admin/dashboard page.

This page was just a combination of the forum listing and the bulk
update requests listing. It got zero hits in the last week.

app/logical/admin_dashboard.rb

@@ -1,17 +0,0 @@
-class AdminDashboard
-  def tag_aliases
-    TagAlias.where(status: "pending").order("id desc")
-  end
-
-  def tag_implications
-    TagImplication.where(status: "pending").order("id desc")
-  end
-
-  def update_requests
-    BulkUpdateRequest.where(status: "pending").order("id desc")
-  end
-
-  def forum_topics
-    ForumTopic.search(category_id: 1).order("id desc").limit(20)
-  end
-end