fix(channels): the Length column shows the sum alone

The "+N without length" line did not fit the 4.5 rem column (88 px of
text in a 56 px cell) and spilled into Tags. The owner wants no such
line there: the cell shows the summed hours, and the channel list no
longer sends a count nothing reads. A playlist's total keeps it.

backend/tests/test_video_length.py

@@ -141,7 +141,8 @@ def test_the_totals_leave_it_out_and_count_the_live_ones(db, make_user, as_user)
     assert channels.status_code == 200
     (ch,) = channels.json()
     assert ch["total_duration_seconds"] == KNOWN_SUM
-    assert ch["live_without_length"] == 2
+    # The channel table shows the sum alone (the count did not fit its column), so it is not sent.
+    assert "live_without_length" not in ch
 
 
 def test_the_duration_filter_and_sort_pass_it_over(db, make_user, as_user):