Turn off CONFIG_RT_GROUP_SCHED in focal derivatives

[Seth Forshee]

BugLink: https://bugs.launchpad.net/bugs/1875665

The following patches set CONFIG_RT_GROUP_SCHED=n in focal for
derivatives, as was done for the master in [1]. linux-aws and
linux-raspi did not ever have this option enabled, so no patches are
required for those kernels.

Thanks,
Seth

[1] https://lists.ubuntu.com/archives/kernel-team/2020-May/109648.html