Delete old "default_guards" state file on startup.

We should have a process to migrate from old state to new state on startup, whenever it makes sense to do so. We should start it out by deleting the default_guards file that will become obsolete once !233 (merged) is merged. We should also delete all the old *.toml files that might be there from back before we changed to json.

The following discussion from !233 (merged) about (default_guards) should be addressed:

• @Diziet started a discussion: (+1 comment)

  We should delete it, IMO. AIUI not migrating it simply means users on this pre-release non-supposedly-actually-secure branch are exposed to a slighly larger guard set, which I think is (i) well within our promises for this revision and (ii) trivial anyway.

  If we leave it lying around, then we will forget about it and eventually early adopters of arti will be using a properly supported arti version but this weird file will still be there.

  ISTM that the other approach to this would be to have a "declare this branch good" flag day and insist that post-1.0 artis delete all pre-arti-1.0 state completely.