sched/fair: Compile out NUMA code entirely when NUMA is disabled

Scheduler code is very hot and every little optimization counts. Instead
of constantly checking sched_numa_balancing when NUMA is disabled,
compile it out.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Change-Id: I7334594fbe835f615a199cfe02ee526135abab06