Fix urlparse import in mrjob.fs.s3

[This commit claims to fix a `urlparse` import](https://github.com/Yelp/mrjob/commit/533f7f5a25424969166aca827b7f4c7c534b3a03) from #672, but it just moves the line later in the file.

So I really fixed it this time. Also removed an unused variable.