改打开题目为当前窗口打开，并且去掉题目单页中的返回选项

template/src/oj/problem/problem.html

@@ -9,9 +9,6 @@
             <li role="presentation">
                 <a href="/problem/{{ problem.id }}/submissions/">我的提交</a>
             </li>
-            <li role="presentation">
-                <a href="/problems/">返回</a>
-            </li>
         </ul>
         {% include "oj/problem/_problem_header.html" %}