Remove assertion from TASK_INIT() macro, since some users of

sys/taskqueue.h may not have includes that define MPASS. It
was useful during testing of r357771, but can be omitted now.
An invalid value of priority will yield only in potential
priority inversion, not a crash.

This fixes compilation of ports/x11/nvidia-driver.

sys/sys/taskqueue.h

@@ -121,7 +121,6 @@ void	taskqueue_thread_enqueue(void *context);
  * Initialise a task structure.
  */
 #define TASK_INIT_FLAGS(task, priority, func, context, flags) do {	\
-	MPASS((priority) >= 0 && (priority) <= 255);		\
 	(task)->ta_pending = 0;					\
 	(task)->ta_priority = (priority);			\
 	(task)->ta_flags = (flags);				\