Remove _task_sleep() from slurm_jobacct_gather.c.

The use in _watch_tasks needs to be removed as the switch to pthread_signal
from pthread_cancel means this will not get interrupted and would keep the
step alive for at least a second, potentially harming throughput.
Since the call to _poll_data() happens after the first timer expires,
this delay turns out to be unnecessary, so we won't be replacing it with
a pthread_cond_timedwait() construct.

The use jobacct_gather_stat_task() is unnecessary since the two locations
this can happen take place after _fork_all_tasks() has setup the tasks,
thus the delay should not be necessary.

Bug 5103.