Revert "scheduler: compute time-average nr_running per run-queue"

This reverts commit 6a8e88aeb641bec5c50b2b2204ef770ac1946c6b.
Revert "Revert "rcu: Yield simpler""

This reverts commit 68c20a2292523b6846515d7ca26ebe93ce6c8a4c.