systemd: drop "nm-sd-utils-core.h" and nm_sd_utils_id128_get_machine()

This was only for unit testing, to check whether our reader
for "/etc/machine-id" agrees with systemd's.

That unit test was anyway flawed, because it actually accesses
the machine-id on the test system.

Anyway. Drop this. Most likely our parser is good enough, and
if we get a bug report with a defect, we can unit test against
that.
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