ARROW-2906: [Website] Remove the link to slack channel

Author: Kenji Okimoto <okimoto@clear-code.com>

Closes #2318 from okkez/remove-slack-channel-link and squashes the following commits:

5606f1fc <Kenji Okimoto> ARROW-2906:  Remove the link to slack channel
diff --git a/_includes/header.html b/_includes/header.html
index e6d7479..a7428675 100644
--- a/_includes/header.html
+++ b/_includes/header.html
@@ -25,7 +25,6 @@
             <li><a href="https://issues.apache.org/jira/browse/ARROW">Issue Tracker</a></li>
             <li><a href="https://github.com/apache/arrow">Source Code</a></li>
             <li><a href="http://mail-archives.apache.org/mod_mbox/arrow-dev/">Mailing List</a></li>
-            <li><a href="https://apachearrowslackin.herokuapp.com">Slack Channel</a></li>
             <li><a href="{{ site.baseurl }}/committers/">Committers</a></li>
             <li><a href="{{ site.baseurl }}/powered_by/">Powered By</a></li>
           </ul>