remove go-simple-db references from go-slog

- ref: https://github.com/GigaOM/legacy-pro/issues/3806
- particularly this [comment](https://github.com/GigaOM/legacy-pro/issues/3806#issuecomment-49641387)
- accompanying config: https://github.com/GigaOM/gigaom/pull/4903

Switch back to:

* `slog_date`
* `slog_class`
* `slog_message`
* `slog_data`