MINOR: task: use LIST_DEL_INIT() to remove a task from the queue

MINOR: task: use LIST_DEL_INIT() to remove a task from the queue

By using LIST_DEL_INIT() instead of LIST_DEL()+LIST_INIT() we manage
to bump the peak connection rate by no less than 3% on 8 threads.
The perf top profile shows much less contention in this area which
suffered from the second reload.