Remove support for internal db-api

[matthieu526-scalingo]

The db-api has been de-provisioned, and a redirection is used. But it means we can remove the support and point to the API only.
It should be done after Scalingo/go-scalingo#514 is done.

[matthieu526-scalingo]

Putting this on hold for now. The behaviour is not the one expected. The switch of endpoint comes with a side effect, where the "prefix" are different. IE /v1 vs /api. It would require keeping both clients, or refactor in a more complex way. This is not worth it for now as this is a temporary situation.