Revert "softirq: Use hotplug thread infrastructure"

This reverts commit 530dd869e5a220a2d500da6d3def389d9ecc82fd.
Revert "rcu: Use smp_hotplug_thread facility for RCUs per-CPU kthread"

This reverts commit ff1a7aba600b921a030c9ade5199233efc6ce662.