systemd: Stop supporting sysvinit compatibility

systemd: Stop supporting sysvinit compatibility

Systemd has worked with sysvinit compatibility but upstream plans to remove
this and hence we will no longer be able to support it. We need to remove this
before the LTS since the support will certainly not be there for the lifetime
of the LTS.

This patch disables that usage mode and removes some of the compatibility logic.
There is more that can be removed, this is meant just to signal to people it is
no longer supported and start that process. Further cleanup can be made over time
in later commits.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>