Remove project bots with no membership

Proposal

There are some project bots whose membership has expired (following !43605 (merged), which sets membership to expire when the project access token expires) but have not been deleted (preceding !45828 (merged), which looks through expired members and deletes them if project bot). So there are project bots that won't get picked up by RemoveExpiredMembershipWorker because they have no membership, and they're just outstanding in the Users table.

We should write a worker that destroys users that are project bots and have no membership. Since this situation is unlikely to keep happening (after !45828 (merged) is merged), this worker doesn't need to be run regularly, just once after !45828 (merged) is merged.