Do not try to remove symlinks for sha1 hash type

The sha1 hash type is the target of symlinks,
not a source of symlinks.

Traceback (most recent call last):
  File "bin/remove-obsolete-new-packages", line 178, in <module>
    remove(hash_cache_path, 'symlink from %s cache' % hash_type)
  File "bin/remove-obsolete-new-packages", line 121, in remove
    os.rmdir(os.path.dirname(path))
  File "/usr/lib/python3.5/posixpath.py", line 148, in dirname
    i = p.rfind(sep) + 1
AttributeError: 'NoneType' object has no attribute 'rfind'