cpu/hotplug: Delete an extraneous kernel-doc description

struct cpuhp_cpu_state has an extraneous kernel-doc comment for @cpu.
There is no struct member by that name, so remove the comment to
prevent the kernel-doc warning:

  kernel/cpu.c:85: warning: Excess struct member 'cpu' description in 'cpuhp_cpu_state'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240114030615.30441-1-rdunlap@infradead.org