wimlib_iterate_dir_tree(): don't checksum unhashed blobs

wimlib_iterate_dir_tree(): don't checksum unhashed blobs

wimlib_iterate_dir_tree() on a modified-but-not-committed image is very
slow because it checksums all unhashed blobs.  This was originally
implemented by commit 681faad85f73 ("wimlib_iterate_dir_tree(): checksum
unhashed blobs"), presumably to make the sha1_hash field always valid.
However, I can't remember a real use case for this.  The current
behavior is causing problems, so let's just revert it and update the
documentation accordingly.

Reported at https://wimlib.net/forums/viewtopic.php?f=1&t=572