Make sure that you don't mark the column where the rownums are inside. This is not really included in the resultset, but in the grid. And the grid is, waht is be exported.
Yes, it's quite easy: Just instead of marking the whole result grid by clicking on the top left corner you mark the first column and then with shift+mouse-click the last column of the result grid and then via mouse-right-click -> Export Results -> CSV File you should get your CSV without the rownum column.
Not sure the version of PL/SQL Dev that you are using (I'm on 10.0.4.1708), but I was also looking for this as I export multiple queries and having to delete the rownum is tedious.
I found that you can remove this under;
Tools > Preferences > Window Types > SQL Window Uncheck "Include row number with export and copy" Apply