Change AssetRemoverTest to use one of the new methods
Change AssetRemoverTest to use one of the new methods

I want to delete some of the tasks that have already been run
including `AssetRemover#remove_organisation_logo`. Changing the tests
to use one of methods I won't remove will let me do that.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall