Revert "[postprocessor/embedthumbnail] Allow mkv to embed thumbnails"

This reverts commit 7360db05b4.

This commit was added as an attempt to fix #6046. Unfortunately, the fix
is completely wrong. As reported on #10359, embedded thumbnails are not
displayed in VLC, and Se7en on IRC reports that the embedded thumbnail
misleads mpv as well.

The correct way is using -attachment of ffmpeg, while the current
run_ffmpeg_multiple_files API can't handle it cleanly.

youtube_dl/postprocessor/embedthumbnail.py

@@ -40,7 +40,7 @@ class EmbedThumbnailPP(FFmpegPostProcessor):
                 'Skipping embedding the thumbnail because the file is missing.')
             return [], info
 
-        if info['ext'] in ('mp3', 'mkv'):
+        if info['ext'] == 'mp3':
             options = [
                 '-c', 'copy', '-map', '0', '-map', '1',
                 '-metadata:s:v', 'title="Album cover"', '-metadata:s:v', 'comment="Cover (Front)"']