Revert enforce existence of composite applicator keyword adjacent to "discriminator" #3137

[skryukov]

This PR reverts changes from #3137

According to 3.1.x version of the spec the discriminator keyword MAY be added to a parent schema definition, example from the spec:

components:
  schemas:
    Pet:
      type: object
      required:
      - petType
      properties:
        petType:
          type: string
      discriminator:
        propertyName: petType
        mapping:
          dog: Dog
    Cat:
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        # all other properties specific to a `Cat`
        properties:
          name:
            type: string
    Dog:
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        # all other properties specific to a `Dog`
        properties:
          bark:
            type: string
    Lizard:
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        # all other properties specific to a `Lizard`
        properties:
          lovesRocks:
            type: boolean

See comment: #2618 (comment)

[Malia1969]

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#2618
desciminador
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[karenetheridge]

+1. when I wrote that PR I clearly didn't think that example through enough :)

That example is buggy (I used it in a test today and got an infinite loop at #/components/schemas/Pet), but we can fix that separately, e.g. in Jason's #2618.