sched/smp: Use the SMP version of idle_thread_set_boot_cpu()

sched/smp: Use the SMP version of idle_thread_set_boot_cpu()

Simplify the scheduler by making the CONFIG_SMP=y version of
idle_thread_set_boot_cpu() unconditional.

Note that idle_thread_set_boot_cpu() is already conditional
on CONFIG_GENERIC_SMP_IDLE_THREAD, which most architectures
select unconditionally on both UP and SMP kernels.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20250528080924.2273858-28-mingo@kernel.org