Demote staticxml backend to support unit tests only

[mmd-osm]

Although the staticxml backend is included in the production build, it is not used productively. It was intended as a way to serve OSM data from an XML file in a read-only scenario. However, memory consumption and performance limit the file sizes significantly. In the end, it is not really suitable for productive use.

Proposed changes:

• Move staticxml* backend to test/ directory for exclusive use in unit tests (DONE: -> Drop support for staticxml backend in production code #376)
• Remove multi backend concept, assume ApiDB as the only (remaining) backend available (DONE: -> Remove multi-backend support #395)
• Remove ENABLE_APIDB compile flag, since we need at least one backend (DONE: -> Remove multi-backend support #395)

Part of https://github.com/mmd-osm/openstreetmap-cgimap/tree/patch/staticxml_test