Removed unused data from the drag event.

This extra data had a side effect, on Linux installations of preventing
some data from being transferred to the drop location if the drag
and drop processes are different.

This is surely a work around for some other deeper problem when communicating between two processes in Linux, but as far as I can tell, the text/url-list data is not used, and eliminating it just happens to fix the problem I'm seeing.  I reported the original problem in atom/atom #3033.