Silently ignore checkbox value fields

[MrCreosote]

The checkbox fields checked_value and unchecked_value are actually completely ignored by the narrative when sending values to an app, and in practice the narrative always sends a 1 for a checked box and a 0 for an unchecked box, regardless of the values of the spec fields.

Furthermore, the NMS silently changes any non-integer field value to 0.

Since the fields are useless and buggy, the current narrative implementation makes intuitive sense anyway, and said implementation been working for 5+ years, just remove the fields from the spec and silently ignore them to avoid breaking current apps with those fields specified.