Revert "hrtimer: Prevent enqueue of hrtimer on dead CPU"

This reverts commit 492d367876978726831d5ec0b925990ced18738f.
Revert "hrtimer: Consider preemption when migrating hrtimer cpu_bases"

This reverts commit fa5ee0618dbeeb2961d865b13b242024c1e2fea0.