x86: only support sparsemem

sparsemem is only one supported, so could remove FLAT_NODE_MEM related,
that is only needed !SPARSEMEM

Signed-off-by: Yinghai Lu <yinghai.lu@sun.com>
Reviewed-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>