Code review comment for ~paelzer/ubuntu-seeds/+git/platform:drop-non-main-from-maas-seed

Discussed with Adam and Dimitri, we'll slightly change the approach.

options:
a) comment them out in supported-maas (as first proposed)
   Pro: false positive goes away
   Con: we do want to express somewhere it is used e.g. to avoid full removal
b) Snaps can use pro, universe is fine, make supported-maas not pull things to main
   Pro: false positive goes away
   Con: MAAS is a good product, keeping in main what we can seems a good choice
c) We could move the "consciously decided to be only universe" dependencies to the community-maas seed
   Pro: false positive goes away
   Pro: we keep signal if e.g. someone deletes things
   Pro: we'd more consciously move things if more would demote in the future