images/archlinux: don't rm root in genimg cleanup

If we fail to umount, then this spams the logfile with numerous
errors, making it difficult to understand the cause of the issue.
This isn't a useful thing to do in the first place since this
directory is mounted.

images/archlinux/genimg

@@ -7,7 +7,6 @@ cleanup() {
     umount -Rf "$root"/boot || true
     umount -Rf "$root" || true
     qemu-nbd --disconnect /dev/nbd0 || true
-    rm -rf "$root" || true
 }
 
 mkdir -p "$arch"