services: openssh: Install OpenSSH in the system profile.

This ensures one can scp to or from the GuixSD machine that uses the
service.

* gnu/services/ssh.scm (openssh-service-type)[extensions]: Add
PROFILE-SERVICE-TYPE extension.
* gnu/system/examples/bare-bones.tmpl <packages>: Remove OPENSSH.
* doc/guix.texi (Using the Configuration System): Adjust accordingly.