Remove unused rake task
Remove unused rake task

The `test:presenters` rake task was added in
3d05d71370eeac1d2806fc0eff0ef86643cd2f4b just in case these tests were
ever needed to be run on their own.

There has never been any usage of this task, so we can delete it.

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