Eliminate the is_path_directory() function.

The is_path_directory() function was only used in two places:

  1. To avoid involving nftw when the target is a file.
  2. To avoid setting (or trying to) a default ACL on a file.

The first one doesn't really matter, and the correct behavior is
tested, so the additional "is it a directory?" check has been
dropped. In the second case, we still don't want to try to set default
ACLs on files, but now the "is it a directory?" check is done at the
call site, where a stat structure is already available.

With those two uses gone, the function has been removed.