x86: remove additional_cpus

remove remainder of additional_cpus logic. We now just listen to the
disabled_cpus value like we did for years. disabled_cpus is always >=
0 so no need for an extra check.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>