Comment parsing: remove HTML attribute validation

Since the community says that a blacklist is not good enough, and I don't have
enough time now to implement a proper whitelist, let's just remove the
attribute validation.

But, nevertheless, we can still communicate in the generated XML if our parser
found an issue with the HTML.  But this bit is best-effort and is specifically
called out in the schema as such.

llvm-svn: 207712