Remove deprecated options for 3.0

Now that master has moved onto 3.0 development, we can remove these deprecated options from 2.x.

- [x] remove deprecated `:error_class` option
- [x] remove deprecated `:hint_class` option
- [x] remove deprecated `:find_options` option
- [x] remove deprecated `:value` option
- [x] remove deprecated `:group_by` and `:group_label` options

Would love code review from @sobrinho at least here before I merge — lots of deletions, especially for the `:group_by` option.