kernel/z_swap: Remove on-stack dummy spinlock

The z_swap_unlocked() function used a dummy spinlock for simplicity.
But this runs afouls of checking for stack-resident spinlocks
(forbidden when KERNEL_COHERENCE is set).  And it's executing needless
code to release the lock anyway.  Replace with a compile time NULL,
which will improve performance, correctness and code size.

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