osd/scrub: remove the 2'nd option for determining 'low load' for scrubbing

osd/scrub: remove the 2'nd option for determining 'low load' for scrubbing

Previously, there were two conditions under which the CPU load was
considered
low enough to allow scrubbing:
- the CPU load was below the configured threshold, or
- the load was below a calculated "daily" average, and lower than the
  15-min average.
That second condition was confusing and surprising, and is now removed.

As the scrubber logic no longer requires the 5m & 15m load averages,
scrub_load_below_threshold() can use the data gathered by the
periodic LoadTracker::update_load_average().

Fixes: https://tracker.ceph.com/issues/71347
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>