Fixed logic error preventing field group 'hide on screen' setting from working

https://github.com/AdvancedCustomFields/acf5-beta/issues/73
Signed-off-by: Elliot Condon <e@elliotcondon.com>
Added support to remove JSON files when deleting a field group

Signed-off-by: Elliot Condon <e@elliotcondon.com>