Unfortunately on a fresh install of 3.0.0.beta16 there is no such trigger in psql that I could drop.
Yet restoring the Backup I made from a 2.9.0.beta12 fails with the same Error.
ERROR: cannot drop column post_id of table bookmarks because other objects depend on it
DETAIL: trigger bookmarks_polymorphic_data_sync on table bookmarks depends on column post_id of table bookmarks
EDIT:
Get it now - deleted trigger in the origin - updateing and restoring backups started working again - thanks for the hint