Avoid unused pg_to_acting_osds call in OSDMap::get_primary_shard for not ec pool

Signed-off-by: Dong Yuan <yuandong1222@gmail.com>
Merge pull request #2667 from yuandong1222/avoid-unused-pg_to_acting_osds-call

Avoid unused pg_to_acting_osds call in OSDMap::get_primary_shard

Reviewed-by: Samuel Just <sjust@redhat.com>