Remove references to annotation ontology.

[gambl]

The spec refers to both the annotation ontology (http://purl.org/ao/) and the open annotation data model (http://www.w3.org/ns/oa#). The open annotation data mode has superseded ao - and we never actually made use of any annotation ontology properties.

[iherman]

Actually: the Web Annotation Data Model, published by the Web Annotation Working Group at the W3C, has superseded the OA specification of the community group, so it is probably better to use that (the editors are the same:-). Note, b.t.w., that this version also includes an alternative serialization of the model in JSON-LD (alongside Turtle), so it may make sense to reuse that verbatim, instead of introducing new terms (about, content).