[bugfix] process account delete side effects in serial, not in parallel

[tsmethurst]

Description

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This pull request refines the account deletion process a little bit, to remove the deleted account's follow(er)s and statuses in serial rather than in parallel. This ought to help prevent worker exhaustion when importing a whole load of domain blocks at once, since previously each deleted status and follow was occupying a worker slot.

Some other small improvements as well, to delete things in a slightly more sensible order.

This PR partially addresses #2350 as well, by removing those calls to Enqueue within a function that was already enqueued. There's still some work to be done there, but it will require a more involved PR.

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.