[BUG] [typescript-fetch] `oneOf` containing `enum` tries to import nonexistent instanceOf function

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?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Using a schema with a oneOf component containing enums leads to leads to broken code where the type tries to import instanceOf functions that aren't generated for enums.

openapi-generator version

6.2.1 and 6.3.0.

OpenAPI declaration file content or url

{
  "openapi": "3.0.0",
  "info": { "title": "Foo", "version": "1.0.0" },
  "servers": [],
  "paths": {},
  "components": {
    "schemas": {
      "Timezone": {
        "oneOf": [
          {
            "description": "(UTC-12:00) International Date Line West",
            "type": "string",
            "enum": [
              "DatelineStandardTime"
            ]
          },
          {
            "description": "(UTC-11:00) Coordinated Universal Time-11",
            "type": "string",
            "enum": [
              "UtcMinus11"
            ]
          },
          {
            "description": "(UTC-10:00) Aleutian Islands",
            "type": "string",
            "enum": [
              "AleutianStandardTime"
            ]
          }
        ]
      }
    }
  }
}

Generation Details

    yarn openapi-generator-cli generate \
        --input-spec {{OPENAPI_SPEC}} \
        --generator-name typescript-fetch \
        --output ./src/api \
        --additional-properties \
    "supportsES6=true,\
    withInterfaces=true,\
    enumNameSuffix=,\
    enumPropertyNaming=UPPERCASE,\
    modelPropertyNaming=camelCase,\
    typescriptThreePlus=true,\
    stringEnums=true\
    "

Steps to reproduce

• Write the schema to a file.
• Run the code typescript-fetch generator
• Try to compile the generated typescript files
• It fails because the type tries to import e.g. instanceOfTimezoneOneOf which doesn't exist.

Start by running the provided OpenAPI schema through the typescript-fetch generator with the listed options, then compile the generated TypeScript files. Trace the generated Timezone oneOf model and its enum variants to find where the nonexistent instanceOfTimezoneOneOf import is introduced. Done means the same schema generates TypeScript that compiles without that import.