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.
del_path can't really fail, if you read the docs, only if the path is somehow invalid or it is not already in the list. Neither of those are really possible if you are doing a LC over elements extracted from the path, so ignoring the return value seems fine to me.