osd/scrub: remove the 'deadline' attribute from the scrub job

osd/scrub: remove the 'deadline' attribute from the scrub job

The scrub job's 'overdue' attribute is no longer calculated -
the only 'scrub is overdue' status remaining after latest
scheduling refactor, is the one performed in PGMap.cc (the
one affecting the 'health warning' status of the cluster).
Thus - there is no longer any reason to maintain any 'deadline'
attribute for the scrub scheduler.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>