Revert "core: Add TODO for testing unplayable-by-backend tracks"

This reverts commit 3a8d896146.

tests/core/test_playback.py

@@ -24,11 +24,6 @@ class TestBackend(pykka.ThreadingActor, backend.Backend):
         super(TestBackend, self).__init__()
         self.playback = backend.PlaybackProvider(audio=audio, backend=self)
 
-    def translate_uri(self, uri):
-        if 'unplayable' in uri:
-            return None
-        return uri
-
 
 class BaseTest(unittest.TestCase):
     config = {'core': {'max_tracklist_length': 10000}}
@@ -659,11 +654,6 @@ class TestUnplayableURI(BaseTest):
         self.assertFalse(success)
 
 
-class TestUnplayableByBackend(BaseTest):
-
-    pass  # TODO
-
-
 class SeekTest(BaseTest):
 
     def test_seek_normalizes_negative_positions_to_zero(self):