DEV: Drop old bookmark columns (#15405)

As per 22208836c5
these are no longer needed, a follow up commit will remove the ignored
columns.

db/post_migrate/20211224010204_drop_old_bookmark_columns.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require 'migration/column_dropper'
+
+class DropOldBookmarkColumns < ActiveRecord::Migration[6.1]
+  DROPPED_COLUMNS ||= {
+    bookmarks: %i{
+      topic_id
+      reminder_type
+    }
+  }
+
+  def up
+    DROPPED_COLUMNS.each do |table, columns|
+      Migration::ColumnDropper.execute_drop(table, columns)
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end