OGM-650 Removing CouchDB dialect from main repo

This removes the CouchDB dialect from _hibernate-ogm_, PR https://github.com/hibernate/hibernate-ogm-contrib/pull/1 adds it to the _hibernate-ogm-contrib_ repo.

There is two minor left-overs which I'm personally willing to accept for now:

* The enum `AvailableDatastoreProvider` used by the testing infra still as a member `COUCHDB` and is used to exclude some tests from running oun CouchDB. We need to generally revisit that concept, i.e. how can dialects (which we may not know in main) choose to not run single tests from the TCK. That'd have to be configured somehow in the dialects themselves rather than centralizing this in main. Not sure about the details, for now the current approach continues to work.
* The RESTEasy version used by the CouchDB dialect is managed in the BOM POM here in the main repo. We don't require it here, though, but I left it there for now to provide one BOM with all the versions for the sake of convinience. In case the contrib repo grows and blossoms, we may consider to establish a separate BOM there later on.