update `downloadCovidDataTotal` and `calculateCovidDataIncrease` based on new JSON structure

[vonStichen]

Daily increase of required metrics (tests and confirmed cases) will newly be included directly in the JSON downloaded from UZIS (along with all required "total" metrics). The remaining daily increase metrics are to be removed (no longer shown in app). As such, the calculateCovidDataIncrease function no longer serves any purpose and should be completely removed.

The downloadCovidDataTotal function should be updated so that it creates an entry also in the collection covidDataIncrease. Click the link to downloadCovidDataTotal function for more details.