Remove dead-code assertion for playlist_duration_sum

There are no current usages, which can be verified with
    git grep playlist_duration_sum

The last usage was removed in 92dcba1e1c,
as per
    git log -S playlist_duration_sum -p

That commit was made over 10 years ago and it seems unlikely than a test
inside a plugin would make use of this special-case assertion. So I
think it is worth removing.

test/test_download.py

@@ -212,10 +212,6 @@ def try_rm_tcs_files(tcs=None):
                     self, len(res_dict['entries']), test_case['playlist_maxcount'],
                     f'Expected at most {test_case["playlist_maxcount"]} entries '
                     f'in playlist {test_case["url"]}, but got {len(res_dict["entries"])}')
-            if 'playlist_duration_sum' in test_case:
-                got_duration = sum(e['duration'] for e in res_dict['entries'])
-                self.assertEqual(
-                    test_case['playlist_duration_sum'], got_duration)
 
             # Generalize both playlists and single videos to unified format for
             # simplicity