cupla: remove "empty" alpaka dir

Follow up to cleaned submodule.
In order to properly make

  git submodule sync

work again, remove the empty dir where the submodule would
usually sit. Otherwise git still stored it.

  https://stackoverflow.com/questions/4185365/no-submodule-mapping-found-in-gitmodule-for-a-path-thats-not-a-submodule

Author: ax3l