x86/smpboot: Remove a stray comment about CPU hotplug

This old comment is irrelavant to the logic of disabling interrupts and
could be misleading. Remove it.

Now, hlt_play_dead() resembles the code that the comment was initially
added for, but, it doesn't make sense anymore because an offlined cpu
could also be put into other states such as mwait.

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230727180533.3119660-2-sohil.mehta@intel.com