Don't complain on missing files.

Removing existing backups is needed if they exist. But the output is
littered with rm complaining about non-existing files. Instead, check if
the file exists, then remove it.

backup/backup

@@ -36,7 +36,7 @@ NR>2 {
         image_url = registry "/" $1 ":" $(i)
         image_file = dest "/" $1 "/" $(i) ".tar"
         printf "Saving %s to %s.\n", image_url, image_file
-        system("rm " image_file)
+        system("[ -f " image_file " ] || rm " image_file)
         if (system("skopeo copy docker://" image_url " docker-archive://" image_file) == 0)
             printf "Backup of %s was successful.\n", image_url
         else {