[AWS S3 Source] Unuspported ignoreBody option

The `ignoreBody` doesn't seem to have any effect. Here is a minimal reproduction steps:

```yaml
apiVersion: sources.knative.dev/v1alpha1
kind: IntegrationSource
metadata:
name: integration-source-aws-s3
spec:
aws:
s3:
arn: "my-bucket"
ignoreBody: true
region: eu-west-1
auth:
secret:
ref:
name: "aws-s3-secret"
sink:
ref:
apiVersion: eventing.knative.dev/v1
kind: Broker
name: default
```

## Expect

Events without the S3 object content as message

## Actuall

Same than without the option, the S3 object content is used as message

version: latest

Start by reproducing the IntegrationSource configuration shown, using an AWS S3 source with ignoreBody set to true. Trace the AWS S3 source handling to determine why the S3 object content remains the message; done means emitted events omit that content when the option is enabled.

Written by the indexing model from the issue text.