check-config.sh: do not check for RT_GROUP_SCHED

Unconditionally checking for RT_GROUP_SCHED is harmful. It is one of
the options that you want inactive unless you know that you want it
active.

Systemd recommends to disable it [1], a rationale for doing so is
provided in

    https://bugzilla.redhat.com/show_bug.cgi?id=1229700#c0.

The essence is that you can not simply enable RT_GROUP_SCHED, you also
have to assign budgets manually. If you do not assign budgets, then
your realtime scheduling will be affected.

If check-config.sh keeps recommending to enable this, without further
advice, then users will follow the recommendation and likely run into
issues.

Again, this is one of the options that you want inactive, unless you
know that you want to use it.

Related Gentoo bugs:
- https://bugs.gentoo.org/904264
- https://bugs.gentoo.org/606548

1: https://github.com/systemd/systemd/blob/39857544ee3794481f165e6ecc5d5a81c65f0ee9/README#L144-L150

Signed-off-by: Florian Schmaus <flo@geekplace.eu>

contrib/check-config.sh

@@ -320,7 +320,7 @@ check_flags \
 	CGROUP_PERF \
 	CGROUP_HUGETLB \
 	NET_CLS_CGROUP $netprio \
-	CFS_BANDWIDTH FAIR_GROUP_SCHED RT_GROUP_SCHED \
+	CFS_BANDWIDTH FAIR_GROUP_SCHED \
 	IP_NF_TARGET_REDIRECT \
 	IP_VS \
 	IP_VS_NFCT \