Revert "workqueue: Backport from upstream"

This reverts commit 4b20c1a334c8b3d85d6899ea644166eb1e18c16d.
Revert "rcu: Use smp_hotplug_thread facility for RCUs per-CPU kthread"

This reverts commit d6951d19e3d62b0d2e46992b3fec52d23bf015dc.
Revert "rcu: Yield simpler"

This reverts commit 388e7a8074b225615f441bcf6f31149310914482.
Revert "softirq: Use hotplug thread infrastructure"

This reverts commit fe6df28e65c20cff015120f87544cb4c684c88d7.