x86, apic: Get rid of apicid_to_cpu_present assign on 64-bit

In fact it's never get used on x86-64 (for 64 bit platform
we use differ technique to enumerate io-units).

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <20091108131645.GD5300@lenovo>
Signed-off-by: Ingo Molnar <mingo@elte.hu>