[Dart] Remove toString() call in enum TypeTransformers

Created by: 0xNF

Fixes #12167 (closed)

The template files for enum TypeTransformers included a data.toString() function that wasn't valid for non-string enums. The result was syntactically incorrect Dart files that wouldn't compile.

This PR removes the method call.

The following specfile can be used to confirm:

specfile

openapi: 3.0.3
info:
  version: "1.1"
  title: Dart Uint8list Demo
servers:
  - url: "localhost"
    variables:
      host:
        default: localhost
paths:
  /item:
    get:
      operationId: GetItem
      description: "Should return an Item"
      responses:
        "200":
          description: items
          content:
            application/json:
              schema:
                $ref: "#components/schemas/item"
components:
  schemas:
    Item:
      type: object
      description: "Some json thing"
      required:
        - strChoice
        - intChoice
      properties:
        strChoice:
          $ref: "#/components/schemas/StrChoice"
        intChoice:
          $ref: "#/components/schemas/IntChoice"
    StrChoice:
      type: string
      enum:
        - FirstChoice
        - SecondChoice
    IntChoice:
      type: integer
      enum:
        - 0
        - 1

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