Clean up and speed up clean_html.
Clean up and speed up clean_html.

Remove a huge swath of unused code.  Generally clean up error handling.

Change drop_invalid_and_upper_utf8_chars() to use a regexp to do the
replacement, rather than manually iterating through the string; it
turns out that re.sub() is a lot faster than ''.join(...for c in s).

trec-kba/streamcorpus-pipeline,trec-kba/streamcorpus-pipeline,trec-kba/streamcorpus-pipeline