Revert "Make PythonTests depend on the ExternalExecutable"

This reverts commit 9416f4e8ab5fe9e1006bf8afd0b9d0ed2655a315.

Change-Id: Id187b98c26e0fb109c26b31fb88bb50d0960d6fb
Reviewed-on: https://gerrit.libreoffice.org/11627
Reviewed-by: Matúš Kukan <matus.kukan@collabora.com>
Tested-by: Matúš Kukan <matus.kukan@collabora.com>