Remove default content/media types

[Sandor]

Currently, if not consumes or produces media types are specified, the generator will add "application/json". While this might come handy, it actually makes a wild assumption and hides a incomplete swagger definition.

We did this in the first place because we faced such definitions with some customer's swagger definitions.

This should be removed in favor of a guidance about how to use custom swagger visitor to fix the definition prior to generation.