Move to osm2pgsql flex output

[lonvia]

The flex output implements most features we need for importing the place table. Therefore we don't need a separate output anymore. The Lua style file would also allow a lot more flexibility than the current json-based configuration. This ticket collects the todo list for the Nominatim side:

• [ ] Move from modify to delete/add behaviour. (Possibily already working with the index_status = 100. Needs tests.)
• [ ] Move tests back to Nominatim.
• [ ] Move hard-coded dropped places from the place_trigger to the flex style file.
• [ ] Supply simple base-style selection in flex config (admin, street, full, etc.).
• [ ] Update style documentation.

There is a working implementation at https://github.com/lonvia/Nominatim/tree/osm2pgsql-flex-output. The open issues on the osm2pgsql side are collected in https://github.com/openstreetmap/osm2pgsql/issues/1311.