fix(appsync): remove 'id' suffix to union definition key (aws#17787)

Removing an unnecessary suffix from union definitions.

Fixes: aws#17771 

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-appsync/lib/schema-intermediate.ts

@@ -381,7 +381,7 @@ export class UnionType implements IIntermediateType {
     if (options.field && !(options.field.intermediateType instanceof ObjectType)) {
       throw new Error('Fields for Union Types must be Object Types.');
     }
-    this.definition[options.field?.toString() + 'id'] = options.field;
+    this.definition[options.field.toString()] = options.field;
   }
 
   /**