Remove download sizes

These numbers must currently be updated manually after a release (but
that never happens in practice).

Most of them could be updated automatically as the files are hosted on
the same server but I personally don't think it's worth the trouble,
and that it's better to remove them.
Merge remote-tracking branch 'remotes/laanwj/2014_08_remove_download_sizes'
Update CSS to display operating systems without download size