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

Simplify the scheduler by making CONFIG_SMP=y code in task_on_cpu()
unconditional.

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/20250528080924.2273858-40-mingo@kernel.org

Author: ingomolnar

kernel/sched/sched.h

@@ -2238,11 +2238,7 @@ static inline int task_current_donor(struct rq *rq, struct task_struct *p)
 
 static inline int task_on_cpu(struct rq *rq, struct task_struct *p)
 {
-#ifdef CONFIG_SMP
 	return p->on_cpu;
-#else
-	return task_current(rq, p);
-#endif
 }
 
 static inline int task_on_rq_queued(struct task_struct *p)