caom2harvester: failure to cleanup skip records when nothing to do

[pdowler]

caom2harvester --validate mode can detect observations that are not in source and records a skip record for the missed deletion.

caom2harvester (incremental mode) will perform the deletion

caom2harvester --skip will "retry" the observation but will fail trying to delete rather than detect that incremental already did it and just cleanup the skip record. Operation should just be silently idempotent.

aside: incremental mode should also remove skip records for deleted observations (it does for inserted/updated).