Remove PubSubHubbub-related columns from accounts table (#16170)

app/models/account.rb

...st_migrate/20210507001928_remove_hub_url_from_accounts.rb

db/schema.rb

spec/controllers/relationships_controller_spec.rb

spec/services/authorize_follow_service_spec.rb

spec/services/batched_remove_status_service_spec.rb

spec/services/block_service_spec.rb

spec/services/favourite_service_spec.rb

spec/services/fetch_remote_status_service_spec.rb

spec/services/process_mentions_service_spec.rb

spec/services/reblog_service_spec.rb

spec/services/reject_follow_service_spec.rb

spec/services/remove_status_service_spec.rb

spec/services/unblock_service_spec.rb

spec/services/unfollow_service_spec.rb