Removing Data Schema Section

[egekorkan]

W3C now allows normative references to JSON Schema (see w3c/strategy#108). See https://www.w3.org/TR/vc-json-schema/#normative-references and also https://w3c.github.io/vc-json-schema-test-suite/. This means that we can finally stop redefining JSON Schema from scratch. This doesn't mean that we do not have to think about it anymore. Some points of clarification:

• As can be seen in the test suite, the VC groups adds assertions on top of JSON Schema's basic rules, which we can do as well, e.g. enum and const cannot be used at the same time. What rules do we have already? What to add?
• Which version of JSON Schema do we refer to?
• I guess we should keep the ontology. How should it be integrated into the toolchain or in the spec?
• We have to still discuss the data mapping concept