[youtube] Do not return `upload_date` for playlists

Closes #2349
Partially reverts #1018
Re-opens #1883

yt_dlp/extractor/youtube.py

@@ -839,7 +839,7 @@ class YoutubeBaseInfoExtractor(InfoExtractor):
             'uploader': uploader,
             'channel_id': channel_id,
             'thumbnails': thumbnails,
-            'upload_date': strftime_or_none(timestamp, '%Y%m%d'),
+            #  'upload_date': strftime_or_none(timestamp, '%Y%m%d'),
             'live_status': ('is_upcoming' if scheduled_timestamp is not None
                             else 'was_live' if 'streamed' in time_text.lower()
                             else 'is_live' if overlay_style is not None and overlay_style == 'LIVE' or 'live now' in badges