remove old formtastic css class and add new ones

this fixes #2734 

After I read some formtastic, I end up on [`wrapper_classes`](https://github.com/justinfrench/formtastic/blob/d107e18f307f90df60e854c0875d225e330a8854/lib/formtastic/inputs/base/wrapping.rb#L35) which uses [`as`](https://github.com/justinfrench/formtastic/blob/d107e18f307f90df60e854c0875d225e330a8854/lib/formtastic/inputs/base/naming.rb#L6) to generate the class attribute of a input element. `as` reflects the name by the ruby input class name. While formtastic and active admin don't have a `DateInput`, `TimeInput` or `DatetimeInput` we can remove `.date`, `.time` and `.datetime` from css. While formtastic as a `DateSelectInput`, `TimeSelectInput` and `DatetimeSelectInput` we should add css for those.