Table wp_bbp_converter_translator should be dropped after a successful import

[beerallica]

That table contains a load of data that is useless after the import process completes successfully. That should be checked at the end of the process and automatically dropped.

[netweb]

Related: #2839 Allow importers to run multiple times and not import duplicates

Per the linked ticket, I've been thinking about how we'd allow subsequent imports. It would enable a site admin to run the import numerous times, this would be beneficial in that a site could continue to use their previous forum right up until switching over to bbPress and a final import of only the differences between the live and test forum needs importing.

Once a site admin no longer needs this table then a new repair tool can added and used to delete this table can be added so the table can be easily deleted at this later stage.