Resolve "Ability to delete identities for user"

What does this MR do?

Allows deletion of identities for users, as an admin, via the API.

Should be enough tests, but also tested locally after adding an identity to a user:

curl --request DELETE \
  --url http://localhost:3000/api/v4/users/22/identities/testing \
  --header 'private-token: FILLMEIN'

Are there points in the code the reviewer needs to double check?

• Are the tests sufficient?
• Is it OK that I moved identity destruction into a service? This ensures that we always use RepairLdapBlockedUserService.
• Is it reasonable to offer just this endpoint without any further endpoints for listing identities? I assume so, as they are listed under a user as well.
• Should we squash these commits? I feel as though they're fine as is, but not sure if you always require a single commit per issue/MR.

Why was this MR needed?

Identities can be deleted via the app, but not the API.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #48739 (moved)