Provide cleanup stub for JSONP cleanups

Prevents global errors for environments that do not terminal script execution when dynamic scripts are removed from the DOM.

This appears to be the case under iOS and Windows phone among others. Note that this issue is not reproducible within PhantomJS as removed scripts have early termination in that environment.
Should this also invoke `originalCallback` function? Seems like it should, since the original function should be restored on window object and usable for calling it.