buildx.sh: fix clean option

./buildx.sh clean results
```./buildx.sh: line 47: ../x11_file_list.txt: No such file or directory```

- fix the use of $mod_dir
  "build_dir/$mod_dir" was evaluated as "build_dir/ Python-2.7" as
  variable mod_dir has leading whitespace

- fix the path of x11_file_list.txt