test: Remove outdated unit_tests dir

Initialy the python unit-tests resided in tests/unit_tests, but later on
they were moved to tests/nose.
During that migration the old dir wasn't deleted. This patch deletes the
old unit-test dir.

Change-Id: I1421282c5379fb38f6e7a502f7f4cd38b3ec82ab
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>