os-build: enable CONFIG_SCHED_CLASS_EXT for RHEL

Phil Auldrequested to merge
prauld/kernel-ark:enable_scx into os-build

Enable sched_ext for RHEL. Since it's enabled for fedora as
well move the config to common. Remove pending-rhel file.

Signed-off-by: Phil Auld pauld@redhat.com

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