osd/scrub: do not clear PG_STATE_REPAIR unconditionally
As we now call clear_pgscrub_state() at the end of each
'Session' state, we must not clear PG_STATE_REPAIR
unconditionally.
Previously - scrubs that reached normal completion, i.e.
reached PgScrubber::scrub_finish(), would have only cleared
that PG flag under specific conditions. That was changed in
previous commits of this PR, and is now fixed.