base-files: Remove checks for upgrade bundle script

Remove unneccessary check for size and content of the upgrade script. It cannot be empty if the tar-command is not buggy and returns 0 for an incomplete extraction. If the script is really empty, then it will execute with error code 0.

Neither is it necessary to to impose limits on the content of the script. Users of the functionality should be free to implement the script any programming language. Remove the check for the she-bang for that reason.