chore: remove systemd reverse dependency by dropping software-properties-common

Through some random machinations, software-properties-common now has a dependency on systemd. This breaks the built docker image as plugins will think systemd is available, so `dokku plugin:install --core` will reference that over sv.

Since we don't manage ppas in Dokku anymore - the only use was for an nginx ppa so we could use a newer Nginx - this can be safely removed.

Also remove some other stuff related to systemd that aren't required anymore.

contrib/images/digitalocean/in_parts/011-docker

@@ -9,7 +9,6 @@ pkgs=(apt-transport-https
   curl
   jq
   linux-image-extra-virtual
-  software-properties-common
 )
 
 echo '--> Updating apt repositories'

contrib/images/digitalocean/in_parts/012-dokku-packages

@@ -15,7 +15,6 @@ pkgs=(apache2-utils
   netcat
   net-tools
 
-  software-properties-common
   parallel
   python3-software-properties
   rsync