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.
Should we really ignore the result of `del_path/1`? In the old code we check that `code:set_path/1` is successful.

Also, as the result of the list comprehension is ignored, we either have to change it to `_ = [...]` or use `list:foreach`.