osd/PeeringState: do not trim pg log past last_update_ondisk

osd/PeeringState: do not trim pg log past last_update_ondisk

Trimming past last_update_ondisk would be really bad, e.g.,
a new interval change would cancel&redo a previous op, and if
we trim past last_update_ondisk, there could be potential
object inconsistencies as log merging won't necessarily be able
to find all divergent entries later (we lost track of the unfinished
op that should really be reverted).

Fixes: https://tracker.ceph.com/issues/44532
Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>