DEV: Ignore reminder_type for bookmarks (PR #14349)

We don’t actually use the reminder_type for bookmarks anywhere; we are just storing it. It has no bearing on the UI. It used to be relevant with the at_desktop bookmark reminders (see fa572d3a7a6a3e107fdbc5f00e04d2bb990ab77d)

This commit marks the column as readonly, ignores it, and removes the index, and it will be dropped in a later PR. Some plugins are relying on reminder_type partially so some stubs have been left in place to avoid errors.