Completely remove Akonadi Relations

This has only ever worked for Kolab resource, no-one else can sync this kind of information. Long-term we want to keep this kind of information in the search index rather than directly in Akonadi database, from where it cannot be reliably synced, especially in case of cross-resource relations.

Since we don't officially maintain public stable API/ABI as of yet, I decided to remove the feature completely instead of just deprecating everything and keeping it around until KPim7. If you are a dev, you may need to rebuild most of your PIM stack.

This mustn't be merged before Relations are removed from everywhere, most notably kdepim-runtime!