Remove x-nullable extension support to support nullable

Created by: jmini

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Description of the PR

As discussed https://github.com/OpenAPITools/openapi-generator/pull/889#issuecomment-416980977, the implementation proposed in https://github.com/OpenAPITools/openapi-generator/pull/889 needs to be adapted.

Swagger-Parser is doing the conversion from x-nullable OASv2 to nullable OASv3. See an example in https://github.com/swagger-api/swagger-parser/issues/810.

This PR removes reading x-nullable extension from our code, because the extension will only be available in OASv3. In v3 having them make no sense as pointed out by @wing328.

There is still an open issue: https://github.com/swagger-api/swagger-parser/issues/820 to have the formData parameter handled in swagger. I added a reference in near to the commented unit-test.