x86/cpu: Remove redundant CONFIG_NUMA guard around numa_add_cpu()

Remove unnecessary CONFIG_NUMA #ifdef around numa_add_cpu() since the
function is already properly handled in <asm/numa.h> for both NUMA and
non-NUMA configurations. For !CONFIG_NUMA builds, numa_add_cpu() is
defined as an empty function.

Simplify the code without any functionality change.

Testing: Build CONFIG_NUMA=n

Signed-off-by: Shivank Garg <shivankg@amd.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20241112072346.428623-1-shivankg@amd.com