fixed test cleanup, some cleanup of commented lines

Author: jgrewe

nixio/test/test_doc_examples.py

@@ -55,7 +55,7 @@ def setUp(self):
     def tearDown(self):
         plt.close("all")
         plt.ioff()
-        if Path.exists(Path("TEST_IMAGE")):
+        if Path.exists(Path(TEST_IMAGE)):
             Path.unlink(Path(TEST_IMAGE), True)
 
     def test_annotations(self):

nixio/util/util.py

@@ -97,7 +97,6 @@ def time_to_str(time):
     :return: string in the form "YYYYMMDDTHHMMSS", where T is the date-time separator
     :rtype: str
     """
-    # dt = datetime.utcfromtimestamp(time)
     dt = datetime.fromtimestamp(time, timezone.utc)
     return dt.strftime("%Y%m%dT%H%M%S").encode("utf-8")