Retire mimicked datasets

[mabijkerk]

Background
In ETSource, we have mimicked datasets for the Netherlands 2016, 2017 and 2018. These datasets served as minimal parents for the regional datasets, which used to have different start years. One RES-region might have had 2016 as a start year, whereas a municipality might have had 2018.

Issue
With #3294 all regional datasets for the Netherlands were updated to start year 2023. There is no use anymore for these mimicked datasets on ETSource, but they still take (a small) effort to maintain for example in #3311.

Solution
We should retire the mimicked datasets.