2573: don't add duplicate newlines when parsing DFXP

The issue was that our DFXP parsing code was treating <br> like a
<span> tag, which was wrong.  It would add "\n" on both sides of the
content (which is always empty), resulting in 2 newlines.

Also added a fix not to call dfxpToMarkdown in the SubtitleList code.
Since the parser already does that, we don't need to do it again.
2574: remove xml namespace from <br> tags