[bugfix] Update poll delete/update db queries

[tsmethurst]

Description

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This pull request updates our db queries a little bit for polls:

• Don't bother checking for db.ErrNoEntries on delete, since this is never returned; use RowsAffected instead. You can never recover from an error inside a transaction, so checking for an ErrNoEntries and continuing would cause the next query to fail anyways.
• Fix choices slice return -- you can't do LIMIT on a Delete using bun, so bun doesn't know that only one row will be deleted, so you have to select into a slice of slices.
• Select fewer columns where possible (v. small performance tweak).

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.