[BUG] Generation of models with invalid imports (aspnetcore)

Created by: riede

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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?
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Description

After generating models for .Netcore 3.1 the models contains unused usings (imports). However, running dotnet build fails because one of the unused usings reference to a namespace that doesn't exist anymore. You can see it in the following image:

The referenced namespace exists when I generate all files. But I only need the models. The import seems to be a relict of generated files (necessary for generating the models) that are removed afterwards while the import still exists

openapi-generator version

❯ openapi-generator version
5.1.0

OpenAPI declaration file content or url

In the following you can find the openapi.json:

openapi.json

{
    "openapi": "3.0.2",
    "info": {
        "title": "TestProject",
        "version": "0.0.1"
    },
    "paths": {
        "/api/Product": {
            "get": {
                "tags": [
                    "Product"
                ],
                "summary": "Product",
                "operationId": "product_api_Product_get",
                "responses": {
                    "200": {
                        "description": "Successful Response",
                        "content": {
                            "application/json": {
                                "schema": {
                                    "title": "Response Product Api Product Get",
                                    "type": "array",
                                    "items": {
                                        "$ref": "#/components/schemas/Product"
                                    }
                                }
                            }
                        }
                    }
                }
            }
        }
    },
    "components": {
        "schemas": {
            "Product": {
                "title": "Product",
                "required": [
                    "id"
                ],
                "type": "object",
                "properties": {
                    "id": {
                        "title": "Id",
                        "type": "string",
                        "format": "uuid"
                    },
                    "machine": {
                        "title": "Machine",
                        "type": "string",
                        "nullable": true
                    }
                }
            }
        }
    }
}

Generation Details

config.json:

{
    "aspnetCoreVersion": 3.1,
    "swashbuckleVersion": "5.0.0",
    "packageName": "TestProject",
    "packageTitle": "TestProject",
    "sourceFolder": ""
}

Steps to reproduce

use the config.json and the openapi.json
run openapi-generator generate -i openapi.json -g aspnetcore --config config.json --global-property models,modelDocs=false

Expected behaviour

The models were generated at least without using TestProject.Converters;. Much better would be a solution where unused usings will be removed.