Legacy code cleanup in Import::DeletePlaceholderUserWorker

We've updated the parameter of Import::DeletePlaceholderUserWorker in milestone 18.0. To follow Sidekiq development guidelines we have created this issue which addresses the next step: removing legacy code from the worker.

The required changes include:

  • Removing the legacy code that uses source_user_id for user deletion.

Additional context is available in the referenced MR.

Edited by 🤖 GitLab Bot 🤖