moby: Remove unneeded systemd cgroup mount

[tsndqst]

This change removes the code that created the /sys/fs/cgroup/systemd mountpoint.
The change was added in an earlier PR that referenced a page in the wiki. The wiki is gone but I believe the error was the one described in this comment.
I do not believe the mountpoint is required anymore.

Also, when running kubernetes on a host with docker running the following errors show up in the kubelet logs. They disappear when the /sys/fs/cgroup/systemd mountpoint is removed.

2025-01-15T07:59:14.160779-06:00 localhost kubelet: E0115 07:59:14.160635    2856 cadvisor_stats_provider.go:522] "Partial failure issuing cadvisor.ContainerInfoV2" err="partial failures: [\"/systemd\": RecentStats: unable to find data in memory cache]"

2025-01-15T07:59:14.857118-06:00 localhost kubelet: W0115 07:59:14.857085    2856 container.go:588] Failed to update stats for container "/systemd": error while statting cgroup v2: [openat2 /sys/fs/cgroup/systemd/pids.current: invalid cross-device link openat2 /sys/fs/cgroup/systemd/memory.stat: invalid cross-device link openat2 /sys/fs/cgroup/systemd/io.stat: invalid cross-device link openat2 /sys/fs/cgroup/systemd/cpu.stat: invalid cross-device link openat2 /sys/fs/cgroup/systemd/cpu.pressure: invalid cross-device link openat2 /sys/fs/cgroup/systemd/memory.pressure: invalid cross-device link openat2 /sys/fs/cgroup/systemd/io.pressure: invalid cross-device link openat2 /sys/fs/cgroup/systemd/hugetlb.2MB.rsvd.current: invalid cross-device link openat2 /sys/fs/cgroup/systemd/rdma.current: invalid cross-device link openat2 /sys/fs/cgroup/systemd/misc.current: invalid cross-device link], continuing to push stats

Testing the changes

• I tested the changes in this PR: YES
• I tested with all three CGROUP_MODE values in /etc/rc.conf

Local build testing

• I built this PR locally for my native architecture, (x86_64-glibc)
• I built this PR locally for these architectures (if supported. mark crossbuilds):
  • aarch64