Clean up the ```._resource``` field in the models

@dciborow submitted PR for azure/partnercenter-cli-extension#186 noting the issue with the syntax and that it violated linting rules.

[Link to comment thread](https://github.com/Azure/partnercenter-cli-extension/pull/186/files#r1123597903)

We need to continue to support out of the box behavior that the CLI gives us inheriting from `msrest.serialization.Model` while having a pythonic and clean way for it to ignore the field when serializing out from the CLI.