Titles are never deleted, even when the pad is deleted

[bakerst-221b]

When a pad is deleted, its title is not deleted from the database.
(Originally discovered by @TheDaemoness)

Steps to reproduce

This is apparent to the user when following these steps:

1. make a pad
2. set a title
3. remove the pad via API
4. recreate the pad under the same pad ID

Superficial analysis

I'm not familiar with the codebase so I hope this is correct:

etherpad-lite does trigger a padRemove hook when a pad is removed.
However, the ep_set_title_on_pad plugin does not implement that hook.