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

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This reverts commit 86650176760d222ef79e2e99c6f5e7b91f181dfd.
Revert "hrtimer: Consider preemption when migrating hrtimer cpu_bases"

This reverts commit 54e32bfb0d986aa2d2a17e3e81ec8759df108fef.