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.
How about the following?
```
    case CurrentPath -- AddedPaths of
        OrigPath ->
            [code:del_path(Path) || Path <- AddedPaths],
            true;
        _ ->
            code:set_path(OrigPath)
    end.
```