x86/Voyager: remove HOTPLUG_CPU Kconfig quirk

Voyager has this Kconfig quirk:

 config HOTPLUG_CPU
 	bool "Support for hot-pluggable CPUs"
	depends on SMP && HOTPLUG && !X86_VOYAGER

But this exception will be moot once Voyager starts using the
generic x86 code.

Signed-off-by: Ingo Molnar <mingo@elte.hu>