x86/hotplug: Remove incorrect comment about mwait_play_dead()

The comment that says mwait_play_dead() returns only on failure is a bit
misleading because mwait_play_dead() could actually return for valid
reasons (such as mwait not being supported by the platform) that do not
indicate a failure of the CPU offline operation. So, remove the comment.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230128003751.141317-1-srivatsa@csail.mit.edu