R1:f97d120c3fe6

When a task is removed from projects, remove its position on proxy columns for those projects

Summary:
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.

Test Plan:
- Added a failing unit test, made it pass.
- Added a task to "X > Milestone 1"…