Revert "nixos/systemd.nix: don’t require online for multi-user.target"

This reverts commit 764c8203b833176d546395a5c1adf193a9ca73f8.

While this is desireable in principle, some of our modules and services
fail during service startup if no network is available don't currently
properly set Wants=network-online.target.

If nothing pulls in this target anymore, systemd won't try to reach it.

We have many VM tests waiting for `network-online.target`, and after
764c8203b833176d546395a5c1adf193a9ca73f8 fail with the following error
message:

```
error: unit "network-online.target" is inactive and there are no pending jobs
```

Most likely, test scripts shouldn't wait for `network-online.target` in
first place (as `network-online.target` says nothing about whether a
service has been started), but instead, the script should wait for the
network ports of the corresponding service to be open.

Let's revert this for now, and re-apply in a draft PR, fixing the tests
before merging it back in.

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