By default, the Caldera Forms REST API will return all fields of a form. This filter can be used to hide fields by ID, type or other conditions. Please keep in mind that the Caldera Forms front-end entry viewer relies on the Caldera Forms REST API. Setting a field not to be returned by the REST API will prevent it from being shown in the front-end entry viewer.
Remove A Specific Field From Caldera Forms REST API Responses
Remove Fields Of A Specific Type From Caldera Forms REST API Responses
Where Does This Code Go?
When using WordPress hooks to customize Caldera Forms or other plugins you should not modify the plugin files, or you will lose your changes when you update the plugin. Instead you should create a small plugin to hold the custom code. It's easy, learn how here.
Technically you can add the custom code to your theme's functions.php, but then you will not be able to change your theme and keep these customizations.