Remove unused nlink

Now that --apparent-size does not use stat.nlink * stat.size, nlink is not used anywhere in du and can be removed from what I can see.

src/du/du.rs

@@ -54,7 +54,6 @@ struct Stat {
     is_dir: bool,
     size: u64,
     blocks: u64,
-    nlink: u64,
     inode: u64,
     created: u64,
     accessed: u64,
@@ -69,7 +68,6 @@ impl Stat {
             is_dir: metadata.is_dir(),
             size: metadata.len(),
             blocks: metadata.blocks() as u64,
-            nlink: metadata.nlink() as u64,
             inode: metadata.ino() as u64,
             created: metadata.mtime() as u64,
             accessed: metadata.atime() as u64,