Revert "Implement iterative NUTS algorithm (#1381)"

This reverts commit 30b0c50.

Author: twiecki

pymc3/tests/test_sampling.py

@@ -112,6 +112,7 @@ def test_constant_named(self):
             assert np.isclose(res, 0.)
 
 
+
 class TestChooseBackend(unittest.TestCase):
     def test_choose_backend_none(self):
         with mock.patch('pymc3.sampling.NDArray') as nd: