Manually clean up paths.

Using code:set_path/1 with very large paths is very slow on larger
projects.  On my mid-sized project, it seems to take around .4s per
call.  Emulating the call with direct path removal (using
code:del_path/1) seems to be quite a lot faster.
Do we have no way to tell what order it should be, or is it just too rare to bother writing different code?