Disable CONFIG_RT_GROUP_SCHED

Breaks RT Sched via rtkit-daemon.

See:
- https://bugzilla.redhat.com/show_bug.cgi?id=1980362
- https://lists.freedesktop.org/archives/systemd-devel/2017-July/039210.html
- https://github.com/util-linux/util-linux/issues/359
- https://docs.kernel.org/scheduler/sched-rt-group.html