Revert "Merge webm formats into mkv if thumbnails are to be embedded (#173)"

This reverts commit 4d971a16b8 by @damianoamatruda
Closes #500

This was wrongly checking for `write_thumbnail`

yt_dlp/YoutubeDL.py

@@ -2590,17 +2590,10 @@ class YoutubeDL(object):
 
                     requested_formats = info_dict['requested_formats']
                     old_ext = info_dict['ext']
-                    if self.params.get('merge_output_format') is None:
-                        if not compatible_formats(requested_formats):
-                            info_dict['ext'] = 'mkv'
-                            self.report_warning(
-                                'Requested formats are incompatible for merge and will be merged into mkv.')
-                        if (info_dict['ext'] == 'webm'
-                                and self.params.get('writethumbnail', False)
-                                and info_dict.get('thumbnails')):
-                            info_dict['ext'] = 'mkv'
-                            self.report_warning(
-                                'webm doesn\'t support embedding a thumbnail, mkv will be used.')
+                    if self.params.get('merge_output_format') is None and not compatible_formats(requested_formats):
+                        info_dict['ext'] = 'mkv'
+                        self.report_warning(
+                            'Requested formats are incompatible for merge and will be merged into mkv.')
 
                     def correct_ext(filename):
                         filename_real_ext = os.path.splitext(filename)[1][1:]