Few people used forum subscriptions (only around 100), and even fewer
people were subscribed to active threads. Most subscriptions were for
old threads that will never be bumped again. The implementation also had
a few problems:
* Unsubscribe links in emails didn't work (they unset the user's
receive_email_notifications flag, but forum subscriptions didn't
respect this flag).
* Some users had invalid email addresses, which caused notifications to
bounce. There was no mechanism for preventing bounces.
* The implementation wasn't scalable. It involved a daily linear scan
over _all_ forum subscriptions looking for any topics that had been updated.
Remove the login reminder page. The meaning of "login reminder" wasn't
clear (it's for recovering a forgotten username) and the functionality
was redundant. The password reset page can already be used to recover
forgotten usernames.
There was also a privacy leak, since the login reminder page could be
used to find out whether a given email is in use on Danbooru.
