remove no-longer-accurate comment

about not overriding execfile on 2.x
because we do override it
fix tb_offset when running a file

either with %run or via `ipython script.py`

since we wrap execfile, the tb_offset needs to be 2 instead of the default (1).

closes #4835
test traceback offset for %run and script
Merge pull request #4858 from minrk/execfile-offset

fix tb_offset when running a file