Remove bz.js library since it is now unused

It was made redundant by https://github.com/mozilla/scrumbugz/commit/c31d0597995dbfef6eeab24ad1281bd3a1351230

There is no further references to the file in the repo, including in the PIPELINE_JS list:
https://github.com/mozilla/scrumbugz/blob/master/settings/base.py#L205