Remove `District` table

The District table should be removed as it does not serve a purpose except for preserving the admin_ID and admin_name from the CSV exposure files when outputting augmented exposure files. However, the assets do not reflect the assets of the districts anymore but rather of the Quadtiles. Thus, the export can be handled with the quadkey for admin_ID and admin_name.

The more important reason to remove the District table and the references to it in the entity table is that for each Quadtile multiple entities are created, one for each district. This is counterproductive for displaying damage results or exposure values as they need to be extra aggregated.