removed unused extension. refs #428

we do not have to add extension to URI.
just remove it would solve our problem.

### note:

correct URL in _show_manage_navigation.scala.html is below:
```xml
        <li class="dropdown"><a class="dropdown-toggle" data-toggle="dropdown" href="#"><img class="hidden-phone-inline" src="@routes.Assets.at("/images/momonga1.png")"/> 参加者管理<b class="caret"></b></a>
            <ul class="dropdown-menu">
                <li><a href="/events/participants/@event.getId()">参加者一覧を表示</a></li>
                <li><a href="/events/participants/csv/@event.getId()">参加者リストを CSV (UTF-8) で出力</a></li>
            </ul>
        </li>
```