target-i386: Remove unused APIC ID default code

The existing apic_id = cpu_index code has no visible effect: the PC code
already initializes the APIC ID according to the topology on
pc_new_cpu(), and linux-user memcpy()s the CPU state (including
cpuid_apic_id) on cpu_copy().

Remove the dead code and simply let APIC ID to to be 0 by default. This
doesn't change behavior of PC because apic-id is already explicitly set,
and doesn't affect linux-user because APIC ID was already always 0.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
