Remove Org.OData.Atom vocabulary

The Org.OData.Atom vocabulary defines the annotation terms that can be used to map properties to Atom elements. This is similar to an "Entity Property Mapping" feature in OData V3.

The rules in OData V3 for how to do the mapping, the semantics and results of the mapping, were pretty complex (which is why we removed them from the OASIS spec). People wanted to be able to map to structured terms, repeating terms, etc.

The vocabulary was introduced as a cleaner, simpler way to specify that properties of the entity can be mapped to some of the defined Atom elements (like title, author, etc.)

However, there isn't really a description of the vocabulary, just a bunch of terms that don't really describe how they are intended to be used. Also, the terms are incomplete (they are missing the "Author" term, for example).

Furthermore, our general intention is that vocabularies, which we expect to be more dynamic, be produced through the ecosystem, rather than through OASIS. The only exceptions to this rule are the Core, Capabilities and Aggregation vocabularies that we defined as part of the [Protocol] and [Aggregation] work products because we needed to normatively reference them.