uglify: remove screw-ie8 option

[XhmikosR]

It seems it has been dropped

[XhmikosR]

Unfortunately, there's no changelog upstream so it's hard to find the exact change. The source code in the latest version doesn't have this option. The only thing I could find is this:

    --ie8                       Support non-standard Internet Explorer 8.
                                Equivalent to setting `ie8: true` in `minify()`
                                for `compress`, `mangle` and `output` options.
                                By default UglifyJS will not try to be IE-proof.