rgw: remove spawned_keys filter from incremental data sync

rgw: remove spawned_keys filter from incremental data sync

the spawned_keys filtering is valid "as long as we don't yield",
according to code comments. however, proper enforcement of the
spawn window necessitates yielding when we exceed that window

the key-based filtering provided by spawned_keys is actually already
satisfied by the call to marker_tracker->index_key_to_marker(), which
also takes completions (either from try_update_high_marker() or
finish()) into account

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