Remove our two borg hosts from the inventory

These are managed services and Ansible doesn't run on them. It got boring writing 'all,!rsync_net,!hetzner_storageboxes' in playbooks and ad-hoc commands, so remove these borg hosts from our inventory.

In the process to accomplish the above, I refactored the roles that set up our borg hosts to run on localhost; seems to work well and should prove more flexible if we were to get another storage box.

host_vars/u236610.your-storagebox.de

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-ansible_ssh_user: "{{ hetzner_storagebox_username }}"