Purge old step data on job requeue

Make sure that old step data is purged when a job is requeued.
Without this logic, if a job terminates abnormally then old step
data may be left in slurmctld. If the job is then requeued and
started on a different node, referencing that old job step data
can result in abnormal events. One specific failure mode is if
the job is requeued on a node with a different number of cores,
and the step terminated RPC arrives later, the job and step
bitmaps of allocated cores can differ in size generating an
abort.
bug 1660