nixos/zerotierone: switch from manually generating the .link file to use the module

Previously, systemd.network.links was only respected with networkd
enabled, but it's really udev taking care of links, no matter if
networkd is enabled or not.

With our module fixed, there's no need to manually manage the text file
anymore.

This was originally applied in 3d1079a2,
but was reverted due to 1115959a causing
evaluation errors on hydra.