Remove Custom FileField hook_field_settings()

In #397578: Uncouple ImageField from FileField Custom Hooks, we removed the ability to make widgets that extend FileField through a set of custom hooks. Instead ImageField and modules like it now extend FileField through CCK and Form APIs. It looks like there's still a lingering API that should have been removed but is still around. I don't think removing this will have any effect on existing modules, because as far as I know ImageField was the only module providing an entirely different data type based on FileField.