[extractors] Remove remnants of sportbox extractor (#11954)

youtube_dl/extractor/extractors.py

@@ -888,10 +888,7 @@ from .spiegeltv import SpiegeltvIE
 from .spike import SpikeIE
 from .stitcher import StitcherIE
 from .sport5 import Sport5IE
-from .sportbox import (
-    SportBoxIE,
-    SportBoxEmbedIE,
-)
+from .sportbox import SportBoxEmbedIE
 from .sportdeutschland import SportDeutschlandIE
 from .sportschau import SportschauIE
 from .srgssr import (