x86, apic: Remove enable_apic_mode callback

The enable_apic_mode() apic callback is never called, so remove it.

Signed-off-by: David Rientjes <rientjes@google.com>
Link: http://lkml.kernel.org/r/alpine.DEB.2.02.1407302352320.17503@chino.kir.corp.google.com


Signed-off-by: H. Peter Anvin <hpa@zytor.com>