[BUG][Java][okhttp-gson] Import mapping for base class not replacing discriminator fields

Bug Report Checklist

• [/] Have you provided a full/minimal spec to reproduce the issue?
• [/] Have you validated the input using an OpenAPI validator (example)?
• [] Have you tested with the latest master to confirm the issue still exists?
• [/] Have you searched for related issues/PRs?
• [/] What's the actual output vs expected output?
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Description

Hi, I see some divergence how the model is created between two different libraries.
For example I have specified in the yaml

schemas:
    BaseRequest:
      discriminator:
        propertyName: "no"
      type: object

    FirsRequest:
      allOf:
      - $ref: '#/components/schemas/BaseRequest'

And I also in the generator in the pom file I specified importMapping:

<importMapping>BaseRequest=my.other.BaseRequest</importMapping>

So in the swagger-codegen or v 3.3.4 when I had import mapping for the parent class that had the discriminator field specified,
in the final model object FirsRequest I was extending my.other.BaseRequest and the discriminator field was not included in the FirstRequest what I expected. But now in v5.4.0 I have the "no" field in the constructor but no field declaration.

I expect that if I have import mapping for the base class I want to extend it without additional fields...

openapi-generator version

5.4.0

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

Related issues/PRs

Suggest a fix

Use the inline OpenAPI schema and the pom.xml importMapping as the reproduction inputs for the Java okhttp-gson generator, comparing behavior between version 5.4.0 and the stated older versions. Inspect the generated FirstRequest model and verify that an imported BaseRequest is extended without a duplicate discriminator constructor field or missing declaration.