modify teacher rank in college statistic page

app/views/colleges/_teacher_rank.html.erb

@@ -1,12 +1,7 @@
 <% if @teachers.present? %>
   <% @teachers.each_with_index do |teacher, index| %>
     <tr>
-      <td class="pl20 pr20">
-      <% if index < 3 %>
-        <img src="/images/educoder/competition/<%= index + 1 %>.png" width="18px" height="22px" class="mt8"/></td>
-      <% else %>
-        <%= index + 1 %>
-      <% end %>
+      <td class="pl20 pr20"><%= index + 1 %></td>
       <td class="color-dark"><a href="<%= user_path(teacher['login']) %>" target="_blank" class="task-hide" style="max-width: 84px;"><%= teacher['real_name'] %></a></td>
       <td><%= teacher['course_count'] %></td>
       <td><%= teacher['shixun_work_count'] %></td>