Remove UTF-8 BOMs

These make it hard to determine which files have mixed CRLF+LF line
endings, because dos2unix and unix2dos strip them. In case other tools
also aren't impressed and since they're not often needed, remove them
where it seems they're not needed.

The BOMs were left for the input files for the HTML and WebVTT parser
tests, where they are obviously intentional.

Removed with `git ls-tree -r --name-only HEAD | xargs bomstrip-files`
and manual inspection of the results.