remove "customizing http request headers" guidance to modify CORS headers in the AWS Console in favor of overrides

**Page**: [`/lib/restapi/authz/q/platform/[platform]`](https://docs.amplify.aws/lib/restapi/authz/q/platform/js/#customizing-http-request-headers)

**Feedback**:

Modifying resources in the AWS Console is not typically recommended as subsequent `amplify push` executions may overwrite these modifications. Instead, we should create or link to guidance to use overrides to modify CORS headers