bulk_delete_user_posts REST API issues

It was originally implemented by:

• https://github.com/openedx/edx-platform/pull/37030

Issues with the original implementation:

• It makes user-facing changes but did not go through community product review
• Documentation incorrectly states that deleted posts are limited to course_id; they actually span across the entire org containing course_id if course_or_org=="org".
• Related to ^, it allows anyone with staff or forum mod role in course-v1:MyOrg+X+Y to bulk-delete posts across all of MyOrg. This is a security flaw.
• Does not use api_doc_tools; documentation can only be viewed in the codebase.
• Does not work with MySQL-backed forums
• Adds utility code which reaches directly in MongoDB-backed forums interface

We are reverting the frontend changes:

• https://github.com/openedx/frontend-app-discussions/pull/818

For the backend, we will do one of the following:

• Minimizing impact by down-scoping the backend API to global staff only: https://github.com/openedx/edx-platform/pull/37401
• Put it behind a feature flag
• Revert the backend API entirely: https://github.com/openedx/edx-platform/pull/37403
• Revert the API in Ulmo but leave it in master