x86/docs: remove noirqbalance param docs

Impact: documentation fix

irqbalance was removed by:

  commit 8b8e8c1b
  Author: Yinghai Lu <yhlu.kernel@gmail.com>
  Date:   Tue Aug 19 20:50:23 2008 -0700

Remove the associated documentation for noirqbalance.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>