Revert "[ffmpeg] Fix embedding subtitles (#9063)"

This reverts commit ccff2c404d7ea9f5b21ede8ae57bb79feec7eb94.

Fixes #10081.

The new approach breaks embedding subtitles into video-only or
audio-only files. FFMpeg provides a trick: add '?' after the argument of
'-map' so that a missing stream is ignored. For example:

opts = [
    '-map', '0:v?',
    '-c:v', 'copy',
    '-map', '0:a?',
    '-c:a', 'copy',
    # other options...
]

Unfortunately, such a format is not implemented in avconv, either.
I guess adding '-ignore_unknown' if self.basename == 'ffmpeg' is the
best solution. However, the example mentioned in #9063 no longer serves
problematic files, so I can't test it. I'll reopen #9063 and wait for
another example so that I can test '-ignore_unknown'.

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