rgw: remove destination shard id from rgw_bucket_sync_pair_info

rgw: remove destination shard id from rgw_bucket_sync_pair_info

the sync_pair is used as input to RGWBucketPipeSyncStatusManager::status_oid()
to generate the per-shard sync status object names

this sync status tracks incremental bucket sync, which reads changes
from a source bucket's bilog shard, and copies objects from the remote
source bucket to the local destination bucket

this doesn't require sync to know anything about the destination bucket
shards, so rgw_bucket_sync_pair_info and status_oid() now only track the
the destination's rgw_bucket instead of rgw_bucket_shard

Signed-off-by: Casey Bodley <cbodley@redhat.com>