Subsume the useful bits of the apertis-customizazions package in the recipes

Most of the bits in the apertis-customizazions package come from a time before the move to Debos and with the recent rebase to Buster it became apparent that some are no longer needed:

• sudoers.d/apertis-customizations: Disables fqdn resolution in sudo, libnss-myhostname makes that unneeded
• systemd/system/etc.mount: can be dropped, a read-only /etc doesn't add much
• systemd/system/run-user.mount: /runis a tmpfs,/run/user/$UIDis another tmpfs, mounting a tmpfs on/run/user` is not needed
• systemd/system/var.mount: was probably meant to mount /var as read-only but uses bind instead of rbind so it was effectively read-write anyway
• systemd/system/etc-rw.service: Made obsolete by dropping etc.mount
• systemd/system/var-rw.service: Switching /var to rw was in fact a no-op
• udev-rules.d/60-apertis-customizations.rules: unneeded on the public recipes, see APERTIS-5749
• systemd/system/tmp.mount: should be made a default in the systemd package, Debian doesn't do it due to backward compatibilty concerns that do not affects us, see APERTIS-5750

The useful bits:

• systemd/system/media.mount: mounting a tmpfs on /media still seems sensible to reduce the flash wear
• systemd/system/apertis-create-homedir.service: we still rely on the lazy homedir creation
• sbin/apertis-create-homedir-if-needed: see above
• sbin/apertis-dev: we still need to be able to switch to GPLv3 tools for development purposes
• tmpfiles.d/apertis.conf: systemd now generates an unique machine-id if /etc/machine-id exists but it is empty