Add a route to delete links

Crejak

A route should be accessible to delete existing links. An possible route scheme would be something like `DELETE /link/<link_type>/<link_id>` (example: `DELETE /link/city_object/1`).

EricBoix

Issue move to [this repository](https://github.com/VCityTeam/Spatial-Multimedia-DB/issues/9). Closing.