x86/cpu: Remove unused and undefined __generic_processor_info() declaration

x86/cpu: Remove unused and undefined __generic_processor_info() declaration

The following revert:

  2b85b3d22920 ("x86/acpi: Restore the order of CPU IDs")

... got rid of __generic_processor_info(), but forgot to remove its
declaration in mpspec.h.

Remove the declaration and update the comments as well.

Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: lenb@kernel.org
Link: http://lkml.kernel.org/r/1505101403-29100-1-git-send-email-douly.fnst@cn.fujitsu.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>