Drop the uneeded systemd unit file for docker0 MTU.
Created by: garloff
The config file /etc/docker/daemon.json is still fully effective and all that's needed. So stop setting the MTU in the docker0 device, as it just adds uneeded complexity/cruft and may be fragile as Jonas rightly pointed out in PR #23.
Signed-off-by: Kurt Garloff kurt@garloff.de