Fix issues with mixins.styl (issues 115 and 120)

I've removed the line that caused the ParseError in stylus. I've removed it because:
  * It was causing issues (see 115 and 120)
  * The mixin that the line contains is not used anywhere that I could find (grep -nr normTransitionMulti*) so it shouldn't even affect anything else
  * I don't know stylus, but that line seems very strange to me (especially the lack of indentation at the beginning)

Maybe a cleaner fix from someone who knows stylus and who can understand what's actually going would be better, but at least this gets it running.