Removed unused parameter in dt_deserializer()

In the file watcher/objects/utils.py, on line 120,
there is an unused parameter:
  def dt_deserializer(instance, val):

I removed that parameter, and modified the test.

Change-Id: Ibc7ab703d37d7f9248a84e41508820453c8954b7
Closes-Bug: #1540521

watcher/objects/utils.py

@@ -117,7 +117,7 @@ def dt_serializer(name):
     return serializer
 
 
-def dt_deserializer(instance, val):
+def dt_deserializer(val):
     """A deserializer method for datetime attributes."""
     if val is None:
         return None

watcher/tests/objects/test_objects.py

@@ -172,10 +172,9 @@ class TestUtils(test_base.TestCase):
 
     def test_dt_deserializer(self):
         dt = timeutils.parse_isotime('1955-11-05T00:00:00Z')
-        self.assertEqual(utils.dt_deserializer(None, timeutils.isotime(dt)),
-                         dt)
-        self.assertIsNone(utils.dt_deserializer(None, None))
-        self.assertRaises(ValueError, utils.dt_deserializer, None, 'foo')
+        self.assertEqual(utils.dt_deserializer(timeutils.isotime(dt)), dt)
+        self.assertIsNone(utils.dt_deserializer(None))
+        self.assertRaises(ValueError, utils.dt_deserializer, 'foo')
 
     def test_obj_to_primitive_list(self):
         class MyList(base.ObjectListBase, base.WatcherObject):