[utils,common] Merge format_srt_time and _subtitles_timecode

format_srt_time uses a comma as the delimiter between seconds and
milliseconds while _subtitles_timecode uses a dot. All .srt examples I
found on the Internet uses a comma, so I use a comma in the merged
version. See http://matroska.org/technical/specs/subtitles/srt.html and
http://devel.aegisub.org/wiki/SubtitleFormats/SRT

youtube_dl/extractor/common.py

@@ -1072,9 +1072,6 @@ class InfoExtractor(object):
     def _get_automatic_captions(self, *args, **kwargs):
         raise NotImplementedError("This method must be implemented by subclasses")
 
-    def _subtitles_timecode(self, seconds):
-        return '%02d:%02d:%02d.%03d' % (seconds / 3600, (seconds % 3600) / 60, seconds % 60, (seconds % 1) * 1000)
-
 
 class SearchInfoExtractor(InfoExtractor):
     """