Remove broken optional debug feature

See the broken test and discussion in #43. The problem is that the original "named function" code is just wrong but the proper fix (the one in #43) is not compatible with PhantomJS yet. We (@tobijat, @mariushoch) decided it's better to remove this feature for now. It's an optional (!) debugging (!) feature anyway. #43 should be kept, rebased and merged when PhantomJS got an update some day (see https://github.com/ariya/phantomjs/issues/10522).