Remove systemd backend support

Why? Because it is dangerous to use. You'd have to run manager with root privileges to be able to spawn systemd services via DBus. There is an option to do the same with session instances of systemd, but that is unpackageable and pretty much unusable in production. We will therefore rely on supervisord, as it's support got much better recently.

manager/knot_resolver_manager/datamodel/templates/config.lua.j2

@@ -12,9 +12,6 @@ nsid.name('{{ cfg.nsid }}_' .. worker.id)
 -- LOGGING section ----------------------------------
 {% include "logging.lua.j2" %}
 
--- SUPERVISOR section -------------------------------
-{% include "supervisor.lua.j2" %}
-
 -- WEBMGMT section ----------------------------------
 {% include "webmgmt.lua.j2" %}