x86, numa: kill useless code to improve code readability
According to x86 boot sequence, early_cpu_to_node() always returns
NUMA_NO_NODE when called from numa_init(). So kill useless code to
improve code readability.
Related code sequence as below:
x86_cpu_to_node_map is set until step 2, so it is still the default
value (NUMA_NO_NODE) when accessed at step 1.