sched: cleanup and simplify CONFIG_SMP and SCHED_DEBUG

JIRA: https://issues.redhat.com/browse/RHEL-112445

Cleanups and simplifications that makes CONFIG_SCHED_DEBUG
always on and so removes that config, and make scheduler
code use CONFIG_SMP at all times, removing the ifdeffery
and UP code paths. Neither will have an effect on RHEL
kernels as both CONFIG_SCHED_DEBUG and CONFIG_SMP are
always set anyway. This will help keep the code in sync
with upstream for future maintenance. Pulled in a few
things from the regular scheduler update to get commits
to apply cleanly.

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