DEV: Ignore reminder_type for bookmarks (#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
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.

app/controllers/bookmarks_controller.rb

@@ -14,7 +14,6 @@ class BookmarksController < ApplicationController
     bookmark = bookmark_manager.create(
       post_id: params[:post_id],
       name: params[:name],
-      reminder_type: params[:reminder_type],
       reminder_at: params[:reminder_at],
       options: {
         auto_delete_preference: params[:auto_delete_preference] || 0
@@ -41,7 +40,6 @@ class BookmarksController < ApplicationController
     bookmark_manager.update(
       bookmark_id: params[:id],
       name: params[:name],
-      reminder_type: params[:reminder_type],
       reminder_at: params[:reminder_at],
       options: {
         auto_delete_preference: params[:auto_delete_preference] || 0