passwd: sync etc/{,g}shadow according to etc/{passwd,group}

Refer to coreos#49 (comment),
Remove bin line in group and passwd
Build FCOS, see logs:
```
systemd.post: Creating group 'bin' with GID 1.
systemd.post: Creating user 'bin' (bin) with UID 1 and GID 1.
systemd.post: /etc/gshadow: Group "bin" already exists.
```
According to @cgwalters 's pointer:

The above log will lead systemd-sysusers (during systemd.post)
exit early before saving the updated `/etc/{passwd,group}` refer
to [code](https://github.com/systemd/systemd/blob/main/src/sysusers/sysusers.c#L820),
and bin user/group will not be saved finally.

The root cause is that `gshadow` is not consistent with group,
`gshadow` is from setup, and we override group according to https://github.com/coreos/fedora-coreos-config/blob/testing-devel/manifests/group.

The `shadow` is also from setup, and is not consistent with
passwd, we should also sync it.

Fix coreos/fedora-coreos-tracker#1525

rust/src/passwd.rs

@@ -337,12 +337,6 @@ fn passwd_compose_prep_impl(
         }
     }
 
-    let shadow_file = format!("{}shadow", dest);
-    let contents = rootfs
-        .read_to_string(shadow_file)
-        .with_context(|| format!("Reading {shadow_file}"))?;
-    println!("==={}===", contents);
-
     Ok(())
 }
 
@@ -395,7 +389,7 @@ fn write_data_from_treefile(
         })
         .with_context(|| format!("failed to write /{}", &target_etc_filename))?;
 
-    // Regernate /etc/{,g}shadow according to usr/lib/{password,group}
+    // Regernate etc/{,g}shadow according to etc/{password,group}
     let db = rootfs.open(target_etc_filename).map(BufReader::new)?;
     let shadow_name = if target == "passwd" { "shadow" } else { "gshadow" };
     let target_etc_shadow = format!("{}{}", dest_path, shadow_name);