kernel: remove kernel/internal/smp.h

There is no need for kernel/internal/smp.h as SOF does not call
z_sched_ipi(). Actually... git log over there has no mention of
z_sched_ipi() anywhere, just arch_sched_ipi().

And include <ksched.h> for source using z_sched_ipi() since
they are using scheduling functions, and would be the correct
file to include.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>