Fixes T11088. When a task is removed from a project, we don't normally delete its column positions. If you accidentally remove a project and then restore the project, it's nice for the task to stay where you put it.

However, we do need to remove its positions in proxy columns to avoid the issue in T11088.

• Added a failing unit test, made it pass.
• Added a task to "X > Milestone 1", loaded workboard, used "Edit Projects" to move it to "X" instead, loaded workboard.
  • Before, it stayed in the "Milestone 1" column.
  • After, it moves to the "Backlog" column.