[IA64] remove staled comments in asm/system.h

With the recent optimization made to wrap_mmu_context function,
we don't hold tasklist_lock anymore when wrapping context id.
The comments in asm/system.h must fall through the crack earlier.
Remove staled comments.

I believe it is still beneficial to unlock the runqueue lock
across context switch. So leave __ARCH_WANT_UNLOCKED_CTXSW on.

Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>