properly remove ssl_dhparam line from nginx config

In the case where no DH parameter is specified, a typo in `assets/init` was causing nginx site config to be left in a corrupt state. Nginx refuses to start:
```
2014/12/02 14:23:11 [emerg] 161#0: directive "ssl_dhparam" is not terminated by ";" in /etc/nginx/sites-enabled/redmine:60
```