Simplify the Idunn configuration values

[bbecquet]

Previously the geocoder and the place service where different services, but they are now managed by the same API, Idunn.

The configuration values could be simplified to reflect that, meaning that we don't need a separate services.geocoder.url value https://github.com/Qwant/erdapfel/blob/master/config/default_config.yml#L31