Remove unused event argument from animateScroll call

While I was writing the unit tests, I found that there were some leftovers from the removal of the `event` argument from `animateScroll`: it was still present in the comments, and in the internal call to `animateScroll` ([L345](https://github.com/cferdinandi/smooth-scroll/blob/master/src/js/smooth-scroll.js#L345), thanks `expect(smoothScroll.animateScroll).toHaveBeenCalledWith([...])`).

NB: I kept this check [under a comment](https://github.com/cferdinandi/smooth-scroll/pull/128/files#diff-83335146a1e9b299863154707cdac74fR76) in the unit testing PR, it should be changed if you plan to merge both PRs.