Revert "units: make systemd-repart.service installable"

Revert "units: make systemd-repart.service installable"

This reverts commit 7e1ed1f3b29162df25064b33dc55ac8cf432bb0b.

systemd-repart is not a user service that should be something people
enable/disable, instead it should just work if there's configuration for
it. It's like systemd-tmpfiles, systemd-sysusers, systemd-load-modules,
systemd-binfmt, systemd-systemd-sysctl which are NOPs if they have no
configuration, and thus don't hurt, but cannot be disabled since they
are too deep part of the OS.

This doesn't mean people couldn't disable the service if they really
want to, there's after all "systemctl mask" and build-time disabling,
but those are OS developer facing instead of admin facing, that's how it
should be.

Note that systemd-repart is in particular an initrd service, and so far
enable/disable state of those is not managed anyway via "systemctl
enable/disable" but more what dracut decides to package up and what not.