core: when switching root remove /run/systemd before executing the binary specified...

core: when switching root remove /run/systemd before executing the binary specified by init=

It's important if the binary specified by the init= boot option is not systemd
otherwise it confuses systemctl that incorrectly assumes that systemd is still
the init system due to the presence of /run/systemd/system.

Also some tools might also check the presence of /run/systemd/private to test
if systemd is running as pid1.