Revert "backup: ensure base snapshots are still available after backup"

This reverts commit d53fbe2474.

The HashSet and "register" function are unnecessary, as we already know
which backup is the one we need to check: the last one, stored as
'last_backup'.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

src/api2/backup.rs

@@ -661,7 +661,6 @@ fn download_previous(
             };
             if let Some(index) = index {
                 env.log(format!("register chunks in '{}' from previous backup.", archive_name));
-                env.register_base_snapshot(last_backup.backup_dir.clone());
 
                 for pos in 0..index.index_count() {
                     let info = index.chunk_info(pos).unwrap();