pid1: do not reset subtree_control on already-existing units with delegation

pid1: do not reset subtree_control on already-existing units with delegation

Fixes #8364.

Reproducer:
$ sudo systemd-run -t -p Delegate=yes bash
# mkdir /sys/fs/cgroup/system.slice/run-u6958.service/supervisor
# echo $$ > /sys/fs/cgroup/system.slice/run-u6958.service/supervisor/cgroup.procs
# echo +memory > /sys/fs/cgroup/system.slice/run-u6958.service/cgroup.subtree_control
# cat /sys/fs/cgroup/system.slice/run-u6958.service/cgroup.subtree_control
memory
# systemctl daemon-reload
# cat /sys/fs/cgroup/system.slice/run-u6958.service/cgroup.subtree_control
(empty)

With patch, the last command shows 'memory'.