sched_ext: idle: Remove unnecessary ifdef in scx_bpf_cpu_node()

There's no need to make scx_bpf_cpu_node() dependent on CONFIG_NUMA,
since cpu_to_node() can be used also in systems with CONFIG_NUMA
disabled.

This also allows to always validate the @cpu argument regardless of the
CONFIG_NUMA settings.

Fixes: 01059219 ("sched_ext: idle: Introduce node-aware idle cpu kfunc helpers")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>