OSD: don't clear peering_wait_for_split in advance_map()

I really don't know why I added this...  Ops can be discarded from the
waiting_for_pg queue if we aren't primary simply because there must have
been an exchange of peering events before subops will be sent within a
particular epoch.  Thus, any events in the waiting_for_pg queue must be
client ops which should only be seen by the primary.  Peering events, on
the other hand, should only be discarded if we are in a new interval,
and that check might as well be performed in the peering wq.

Fixes: #6681
Signed-off-by: Samuel Just <sam.just@inktank.com>