riscv: smp: Remove outdated comment about disabling preemption

riscv: smp: Remove outdated comment about disabling preemption

Commit f1a0a376ca0c ("sched/core: Initialize the idle task with
preemption disabled") removed a call to preempt_disable(), but not the
associated comment. Remove the outdated comment.

Fixes: f1a0a376ca0c ("sched/core: Initialize the idle task with preemption disabled")
Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>
Link: https://patch.msgid.link/20260204-riscv-smp-comment-update-2026-01-v1-1-8b77aa181530@iscas.ac.cn
Signed-off-by: Paul Walmsley <pjw@kernel.org>