librem5_defconfig: Remove CONFIG_RT_GROUP_SCHED
From systemd README:
We recommend to turn off Real-Time group scheduling in the kernel when using systemd. RT group scheduling effectively makes RT scheduling unavailable for most userspace, since it requires explicit assignment of RT budgets to each unit whose processes making use of RT. As there's no sensible way to assign these budgets automatically this cannot really be fixed, and it's best to disable group scheduling hence.
Disabling it allows apps like JACK to run with real-time scheduling.
It's disabled in Debian's and Arch's kernels as well.