Deleted chargers are not removed from DB

[johan12345]

If a charging station is removed from the data source, but is still present in EVMap's local DB, EVMap will keep it until the cache limit configured for the data source (24h for GoingElectric, significantly longer for the others) expires or the cache is cleared. If we receive a complete complete ChargepointList response from the API, but the DB contains more chargers for the same region and filter settings, we should delete the extra chargers from the DB directly.

reported in #363

[johan12345]

aacd7c6 addresses this for data sources that use the fullDownload mechanism (OSM, Nobil). We still need to implement a solution for the backend-based data sources.