kernel: Streamline z_is_thread_ready()

The check for an active timeout in z_is_thread_ready() was originally
added to cover the case of a sleeping thread. However, since there is
now a bit in the thread state that indicates if the thread is sleeping
we can drop that superfluous check.

Making this change necessitates moving k_wakeup()'s call to
z_abort_thread_timeout() so that it is within the locked
_sched_spinlock section to ensure that we do not end up with
a stray thread timeout in the timeout list.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>