fix: remove aws_iam directive incorrectly added to interfaces

[svidgen]

Description of changes

#3270 adds the @aws_iam directive to graphql custom types so they can be used in custom operations under the IAM auth mode. However, the change also inadvertently adds the directive to graphql interface types, which is incorrect. Interfaces can't have these directives applied to them.

This change fixes that.

CDK / CloudFormation Parameters Changed

Issue #, if available

Description of how you validated changes

Checklist

• PR description included
• yarn test passes
• E2E test run linked
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• Any CDK or CloudFormation parameter changes are called out explicitly

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

[svidgen]

E2E test run: https://us-east-1.console.aws.amazon.com/codesuite/codebuild/594813022831/projects/amplify-category-api-e2e-workflow/batch/amplify-category-api-e2e-workflow:4b02af60-fc33-42ab-876f-8365ccf84b02?region=us-east-1 ❌

[svidgen]

Post-merge from main: https://us-east-1.console.aws.amazon.com/codesuite/codebuild/594813022831/projects/amplify-category-api-e2e-workflow/batch/amplify-category-api-e2e-workflow:3387d788-f755-4b30-8009-20aec1d4e187?region=us-east-1 ✅

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