Remove database migration scripts.

These were used to migrate from MALMan1 to MALMan2, and are no longer
needed as MALMan1 is now abandoned.
Remove TODO file.

All todo items were merged into the wiki at
<http://discus.voidwarranties.be/index.php/MALMan#To_do>.
I propose to use github issues for concrete bugs and feature requests, and
use the VoidWarranties wiki to keep track of broader todo items.