ZOOKEEPER-3648: remove Hadoop logo in the ZooKeeper documentation

I have removed hadoop logo from header.html present in zookeeper-documentation and also the image hadoop-logo.jpg from images.

Please do let me know if I have missed anything.

Author: ravowlga123 <ravowlga@gmail.com>

Reviewers: Enrico Olivelli <eolivelli@apache.org>, Justin Ling Mao <maoling199210191@sina.com>

Closes #1179 from ravowlga123/ZOOKEEPER-3648

zookeeper-docs/src/main/resources/markdown/html/header.html

@@ -19,9 +19,6 @@
         <a href="http://www.apache.org/">Apache</a> &gt; <a href="http://zookeeper.apache.org/">ZooKeeper</a>
     </div>
     <div class="header">
-        <div class="grouplogo">
-            <a href="http://hadoop.apache.org/"><img class="logoImage" alt="Hadoop" src="images/hadoop-logo.jpg" title="Apache Hadoop"></a>
-        </div>
         <div class="projectlogo">
             <a href="http://zookeeper.apache.org/"><img class="logoImage" alt="ZooKeeper" src="images/zookeeper_small.gif" title="ZooKeeper: distributed coordination"></a>
         </div>