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
fa572d3a7a)
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.
41e19adb0d