[init] service file: drop no longer available ControlGroup= option

Support for that was dropped in systemd 205[1]. To be able to get RT
priority regardless of the runtime control groups (Linux-specific),
there's a precedent solution for corosync[2], should it be needed.

[1] https://lists.freedesktop.org/archives/systemd-devel/2013-July/011679.html
[2] https://github.com/corosync/corosync/pull/231

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>