Don't ping search engines after sitemap generation

The Bing sitemap submission was removed in sitemap_generator 6.3.0 [1],
and Google has also deprecated the ping and we're getting an error when
deploying:

```
Ping failed for Google: #<OpenURI::HTTPError: 404 Sitemaps ping is
deprecated.
```

Since those were the only two search engines we were pinging in the
past, we're removing the code that did so.

[1] Pull request 408 in https://github.com/kjvarga/sitemap_generator

config/sitemap.rb

@@ -52,8 +52,4 @@ Tenant.run_on_each do
       end
     end
   end
-
-  unless Rails.env.development? || Rails.env.test?
-    SitemapGenerator::Sitemap.ping_search_engines
-  end
 end