Add unretweet TwAPI endpoint and cleanup AP.unannounce

lib/pleroma/web/activity_pub/activity_pub.ex

lib/pleroma/web/activity_pub/transmogrifier.ex

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex

lib/pleroma/web/router.ex

lib/pleroma/web/twitter_api/twitter_api.ex

lib/pleroma/web/twitter_api/twitter_api_controller.ex

test/web/activity_pub/activity_pub_test.exs

test/web/twitter_api/twitter_api_controller_test.exs

test/web/twitter_api/twitter_api_test.exs