units/systemd-repart: stop pretending that root config is executed in the initrd

units/systemd-repart: stop pretending that root config is executed in the initrd

I have a system with /usr/lib/repart.d/50-root.conf with GrowFileSystem=yes.
The partition wouldn't be resized in the initrd, because
ConditionDirectoryNotEmpty=|/sysusr/usr/lib/repart.d was evaluated very early,
before /sysroot was mounted. There was no ordering dependency between
systemd-repart.service and sysroot.mount. (There was After=initrd-usr-fs.target,
but it seems to be only referred to by systemd-fstab-generator, which in my
case doesn't even run, because there's no fstab.)

But in fact, we neeed to run systemd-repart in the initrd only in limited
circumstances: when we need to create the root device based on config under
sysusr.mount. If there is config on the root device, it can be executed in
the host system, early during boot. Thus, let's remove the condition on
/sysroot/…. Without an ordering dependency on sysroot.mount, it was subject to
a race condition anyway. (A race condition with a low probability of "winning",
because systemd-repart.service has no dependencies, but sysroot.mount requires
a device to be detected and the mount to happen.)

The other problem was that systemd-repart.service didn't have the ordering wrt.
initrd-switch-root.target, so it was subject to the same race condition that
was fixed for other units in 7c0e2b555968d70ac563a37e32a6931ee90961a6. (If the
systemd-repart.service/stop job is slow, we could end up not restarting
systemd-repart.service in the host system.)

With the changes here, I see systemd-repart.service/start running twice:
in the initrd it is skipped because the conditions fail, and then in the
host system it runs normally.

Note: support for /sysroot is retained in systemd-repart code. I don't see a
strong reason to remove it, since it may still be useful to people invoking
repart in the initrd in other circumstances.