Werk #16156: downtimes: site_id only required when deleting downtimes by_id

We recently introduced a change with werk 15966 which means that the site_id is now required when deleting downtimes via the REST API. However, only the 'by_id' option leaves the downtime selection ambigious, while the options 'params' or 'query' already have the ability to pinpoint the downtimes requested, therefore, the site_id should not be required in these cases.

This werk addresses this oversight and introduces a change where the site_id is only required when deleteing by_id.

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