Correct allOf with only one child schema (no discriminator)

[wing328]

cc @OpenAPITools/generator-core-team

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[rivancic]

Why would I need to use discriminator if I only have one reference in allOf. In this case it's clear that only this single referenced
schema can be a parent schema and in Java it should extend this single parent schema.

For schema

    ResponseRefSchema:
      description: >-
        TestInformation should be ResponseRefSchema parent even without discriminator 
      type: object
      allOf:
        - $ref: "#/components/schemas/TestInformation"

result should be

public class ResponseRefSchema extends TestInformation  {
...

Logic in ModelUtils#getParentName() should be like following:

If (only one interface) then 
  hasAmbiguousParents = false;

or

if (only one interface) then
  return parentName;