Remove unneeded Formtastic overridden method

Looking at Formtastic the `boolean_checked?` method was reverted and then added back since `2.2.0`

I think we can rely on Formtastic `checked?` method [see there](https://github.com/justinfrench/formtastic/blob/master/lib/formtastic/inputs/boolean_input.rb#L104)

Fix #2950