Remove unneeded Formtastic overridden method


Looking at Formtastic the `boolean_checked?` method was reverted and then added back since `2.2.0`

I dont see any reason to override `checked?` method, since it's being overridden as an exact copy ([see there](https://github.com/justinfrench/formtastic/blob/master/lib/formtastic/inputs/boolean_input.rb#L104))

Fix #2950