treewide: sysupgrade: use $UPGRADE_BACKUP to check for backup

Now that $UPGRADE_BACKUP is set conditionally there is no need to check
the $UPGRADE_OPT_SAVE_CONFIG anymore. All conditions can be simplified.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit a858db3136)

package/base-files/files/sbin/sysupgrade

@@ -371,7 +371,6 @@ else
 		$backup_attr
 		\"command\": $(json_string "$COMMAND"),
 		\"options\": {
-			\"save_config\": $SAVE_CONFIG,
 			\"save_partitions\": $SAVE_PARTITIONS
 		}
 	}"