AppImageUpdate: [rewrite] Do not leave .zs-old files around

When the file was already 100% downloaded and up to date and the user then tries to update it, a .zs-old file is left behind:

me@host:~$ ls
appimaged-x86_64.AppImage	  Desktop    Downloads	Pictures
appimaged-x86_64.AppImage.zs-old  Documents  Music	Videos
me@host:~$ rm appimaged-x86_64.AppImage.zs-old 
me@host:~$ ls
appimaged-x86_64.AppImage  Desktop  Documents  Downloads  Music  Pictures  Videos
me@host:~$ Downloads/appimageupdate-x86_64.AppImage appimaged-x86_64.AppImage
Starting update...
Updating from generic server via ZSync
Update URL: https://github.com/AppImage/AppImageKit/releases/download/continuous/appimaged-x86_64.AppImage.zsync
zsync2: appimaged-x86_64.AppImage found, using as seed file
zsync2: Reading seed file: appimaged-x86_64.AppImage
zsync2: Usable data from seed files: 100.000000%
zsync2: Renaming temp file
zsync2: Fetching remaining blocks
zsync2: Verifying downloaded file
zsync2: checksum matches OK
zsync2: used 245760 local, fetched 0
100% done...
Update successful!
me@host:~$ ls
appimaged-x86_64.AppImage	  Desktop    Downloads	Pictures
appimaged-x86_64.AppImage.zs-old  Documents  Music	Videos