Do not enable SSI with SSL

When SSI is enabled for site, nginx will modify Content-Lenght
header. This will break horizon file transfer from different
object storages.

Change-Id: I265966291a77ae49d09cededa89d434513972c5b
Related-Prod: PROD-24932
diff --git a/nginx/files/_ssl.conf b/nginx/files/_ssl.conf
index cd30666..7bc7bd2 100644
--- a/nginx/files/_ssl.conf
+++ b/nginx/files/_ssl.conf
@@ -8,7 +8,6 @@
   {%- set cert_file=site.ssl.get('cert_file', '/etc/ssl/certs/{0}.crt'.format(site.host.name)) %}
   {%- set chain_file=site.ssl.get('chain_file', '/etc/ssl/certs/{0}-with-chain.crt'.format(site.host.name)) %}
 
-  ssi on;
   ssl on;
   ssl_session_cache {{ site.ssl.get('session_cache', 'shared:SSL:10m') }};
   ssl_session_timeout {{ site.ssl.get('session_timeout', '10m') }};