Issue 5942 - don't create Gecko update manifests

sitescripts/extensions/template/updates.json

-{
-  "addons":
-  {
-    {%- for extension in extensions %}
-    {{extension.extensionID|json}}:
-    {
-      "updates":
-      [
-        {
-          "version": {{extension.version|json}},
-          "update_link": {{extension.updateURL|json}}
-          {%- if extension.compat -%}
-          ,
-          "applications":
-          {
-            {%- for app in extension.compat %}
-            {{app.id|json}}:
-            {
-              "strict_min_version": {{app.minVersion|json}}
-            }
-            {{- "," if not loop.last else "" }}
-            {%- endfor %}
-          }
-          {%- endif %}
-        }
-      ]
-    }
-    {{- "," if not loop.last else "" }}
-    {%- endfor %}
-  }
-}