How do I reversibly remove_foreign_key on a table when the column doesn't match the table?

Use revert:

def change
  revert do
    add_reference :users, :seek_asset_type, foreign_key: { to_table: :asset_types }
  end
end

Another way is to revert a migration by its name:

def change
  revert AddAssetTypeReferenceToUsers 
  # I made up this migration name, 
  # so please fill in the appropriate name
end