Remove config option database.DBexport

[DL6ER]

What does this implement/fix?

Remove config option database.DBexport. Its implementation was broken by design as its value was always overwritten by the condition (database.maxDBdays > 0). Replace it with exactly this condition and document the behavior in the config file.

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Related issue or feature (if applicable): Fixes #1853

Pull request in docs with documentation (if applicable): N/A

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[github-actions[bot]]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[DL6ER]

Rebased on latest development-v6

[github-actions[bot]]

Conflicts have been resolved.