services: web: Remove default certificate and key files for nginx.

If nginx is configured with a ssl-certificate file, and ssl-certificate-key,
it will fail to start unless these exist. To avoid this happening, change the
default to #f.

* gnu/services/web.scm (<nginx-server-configuration>)
  [ssl-certificate,ssl-certificate-key]: Set the defaults to #f.
* gnu/tests/web.scm (%nginx-servers): Remove redundant
  nginx-server-configuration fields.
* doc/guix.texi (Web Services): Update examples and documentation.