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.
it doesn't happen at all in the current codebase thatI can tell.  if the code it complex and the case is rare, it seems better to hold off until there's a measured need for it.