Hide navigation to newsletter (#419)

For a long time, it seems that form is broken with Uncaught
ReferenceError and Failed to load resource 404, so hide it until
appropriately handled by admin.

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: kenhys

views/layout.erb

@@ -154,7 +154,7 @@
                 </a>
                 <ul class="dropdown-menu">
                   <li><a href="/blog/">Blog</a></li>
-                  <li><a href="/newsletter">News Letter</a></li>
+                  <!-- <li><a href="/newsletter">News Letter</a></li> -->
                   <li><a href="https://www.cognitoforms.com/Fluentecosystem/FluentEcosystemSurvey">Feedback</a></li>
                   <li><a href="https://store.cncf.io/collections/fluentd">Stickers / T-Shirts / Parkas</a></li>
 		              <li><a href="/contributing">Contributing</a></li>