Remove assertion and confirmation

[pfrazee]

• Remove:
  • app.bsky.graph.assertion
  • app.bsky.graph.confirmation
  • app.bsky.graph.assertMember
  • app.bsky.graph.assertCreator

We think these are the right idea but they're not being used yet, so they can come back in the future

[dholms]

fixed https://github.com/bluesky-social/atproto/pull/658