Remove default application/json header from BulkImports::Clients::HTTP

What does this MR do and why?

This MR removes the default Content-Type header with the value of application/json from the BulkImports:: Clients::HTTP class.

This change was needed to fix the Invalid content type bugfunctional with GSP when proxy_download is set to false.

Relates to: https://gitlab.com/gitlab-org/gitlab/-/issues/383767

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Checkout the bmarjanovic-georgekoltsov/bulk-imports-http-client-header branch
2. Configure the object storage
   • google-cloud-storage-gcs
   • amazon-s3
3. Set proxy-download to false in the config/gitlab.yml file
4. Validate that import works successfully with both OS providers.

MR acceptance checklist

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