The merge request counter in the UI was not decreasing when a merge
request was deleting.
This was just due to the cache not being
refreshed on a delete action.
fixes: https://gitlab.com/gitlab-org/gitlab-ce/issues/44458
When the target is deleted, todos are destroyed, but we did not reset the todo
cache for users with todos on the deleted target. This would only update after
the next time the todo cache was updated for that user.
