[utils] Remove codec2ext

This function is orignally used for determining file extensions of DASH
formats. Now in DASH, ext is determined by mime_type. See #8766 for more
information.

youtube_dl/utils.py

@@ -1893,22 +1893,6 @@ def mimetype2ext(mt):
     }.get(res, res)
 
 
-def codec2ext(codec):
-    codec_type = codec.split('.')[0]
-
-    # Leave the return value None for unknown values as codec_type
-    # is not a good fallback for file extensions
-    return {
-        'avc1': 'mp4',
-        'avc2': 'mp4',
-        'avc3': 'mp4',
-        'avc4': 'mp4',
-        'mp4a': 'm4a',
-        'vorbis': 'webm',
-        'vp9': 'webm',
-    }.get(codec_type)
-
-
 def urlhandle_detect_ext(url_handle):
     try:
         url_handle.headers