custom-scripts/scripts/pre_remove_exceptions: always use --allow-remove-essential

Even with --allow-remove-essential, apt will never remove Essential:yes or Protected:yes packages which were not explicitly asked to be removed:

  15:00 < juliank> So note that it can only remove essential packages specified as an argument
  15:01 < juliank> josch: we call it the "tech youtuber safety" feature
  15:01 < josch> so "apt-get --allow-remove-essential foo" will never remove any package that is essential:yes or protecte:yes other than "foo"
  15:01 < juliank> indeed