Remove server_tokens setting

Revert this if we decide this is a security issue, but we report the version through the API as well

jupyterhub/handlers/pages.py

@@ -423,7 +423,6 @@ class AdminHandler(BaseHandler):
             sort={s: o for s, o in zip(sorts, orders)},
             allow_named_servers=self.allow_named_servers,
             named_server_limit_per_user=self.named_server_limit_per_user,
-            server_tokens=self.settings.get('server_tokens', True),
             server_version='{} {}'.format(__version__, self.version_hash),
         )
         self.finish(html)