[nova][ptg] drop the shadow table concept

CERN reported two issues with archive_deleted_rows CLI: * When one record gets inserted into the shadow_instance_extra but didn't get deleted from instance_extra (I know this is in a single transaction but sometimes it happens), needs manual cleanup on the database * Also there could be two cells running this command at the same time fighting for the API db lock,

TODOs: * tssurya to report bugs / improvements on archive_deleted_rows CLI based on CERN's experience with long table locking * mnaser to report a wishlist bug / specless bp about one step db purge CLI which would skip the shadow tables

Cheers, gibi