kernel: Remove obsolete _mailbox field

Removes the _mailbox from the k_mbox_msg structure. This field
is not used and only existed for legacy API support while Zephyr
was transitioning from the split microkernel/nanokernel to the
current unified kernel design.

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

include/zephyr/kernel.h

@@ -4685,8 +4685,6 @@ static inline uint32_t z_impl_k_msgq_num_used_get(struct k_msgq *msgq)
  *
  */
 struct k_mbox_msg {
-	/** internal use only - needed for legacy API support */
-	uint32_t _mailbox;
 	/** size of message (in bytes) */
 	size_t size;
 	/** application-defined information value */