Remove references to the unsupported config/application.yml

Remove references to the unsupported config/application.yml

The application now refuses to start when config/application.yml
exists, and settings live in config/settings.yml. Drop the macOS note
that asked people to edit GPX directories in that file (trace files
are kept by Active Storage, under the Rails root with the example
storage configuration) and point the bbox comments in the map and
tracepoints controllers at config/settings.yml.