Drop installation notes for Debian Jessie (#10642)

Jessie has not received security updates for at least three years. See https://www.debian.org/releases/jessie/

docs/_sidebar.md

@@ -30,7 +30,6 @@
   * [Equinix Metal](/docs/equinix-metal.md)
   * [vSphere](/docs/vsphere.md)
 * [Operating Systems](docs/bootstrap-os.md)
-  * [Debian](docs/debian.md)
   * [Flatcar Container Linux](docs/flatcar.md)
   * [Fedora CoreOS](docs/fcos.md)
   * [OpenSUSE](docs/opensuse.md)

docs/debian.md

-# Debian Jessie
-
-Debian Jessie installation Notes:
-
-- Add
-
-  ```ini
-  GRUB_CMDLINE_LINUX="cgroup_enable=memory swapaccount=1"
-  ```
-
-  to `/etc/default/grub`. Then update with
-
-  ```ShellSession
-  sudo update-grub
-  sudo update-grub2
-  sudo reboot
-  ```
-
-- Add the [backports](https://backports.debian.org/Instructions/) which contain Systemd 2.30 and update Systemd.
-
-  ```ShellSession
-  apt-get -t jessie-backports install systemd
-  ```
-
-  (Necessary because the default Systemd version (2.15) does not support the "Delegate" directive in service files)
-
-- Add the Ansible repository and install Ansible to get a proper version
-
-  ```ShellSession
-  sudo add-apt-repository ppa:ansible/ansible
-  sudo apt-get update
-  sudo apt-get install ansible
-  ```
-
-- Install Jinja2 and Python-Netaddr
-
-  ```ShellSession
-  sudo apt-get install python-jinja2=2.8-1~bpo8+1 python-netaddr
-  ```
-
-Now you can continue with [Preparing your deployment](getting-started.md#starting-custom-deployment)