When disk cache files were removed, directories stayed in place.

Remove directories when all files are removed from them.

This is probably best solution I can come up with. I don't quite like, that directories have to be traversed towards higher level, because if somehow condition STREQ(path, seq_disk_cache_base_dir() fails and BLI_dir_is_empty(path) fails as well, this can result in rm -rf /.
Alternative would be to track status during initial cache traversing and storing empty branches, that can be removed at once. Which would help when directory is not removed for some reason even when no cache files aren't present.