tests/kernel/smp: Remove test_wakeup_pending_threads case

This case was predicated on a mistake.  The behavior of k_wakeup() has
always been NOT to wake up threads that are "pending" on a wait queue,
only ones blocked on a timeout in k_sleep().  As written, this test
case could never pass.

(Really there's no good reason for that.  It seems reasonable to me to
expect wakeup to work symmetrically, and the docs are sort of
ambiguous on the subject.  But the code in k_wakeup() is clear:
threads flagged pending get an early exit and the call becomes a
noop.)

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>