osd: remove build_past_intervals_parallel

osd: remove build_past_intervals_parallel

This was added in the dumpling era to handle the case where we have lots
of PGs with stale or missing past_intervals.  These days, we update
PastIntervals as we inject new makes (via advance_map), when we
learn about new PGs via notify with get the PastIntervals at the same time,
and during PGCreate handling we have a separate path in
build_initial_pg_history.  This code is obsolete.

It also reaches deep into the PG class where it shouldn't!

Signed-off-by: Sage Weil <sage@redhat.com>