Save Forms by Default & Remove Setting

In an effort to [remove options that users rarely use](http://coenjacobs.me/taking-features-away-woocommerce-2-1/) to make the experience better for everyone else I've removed the  `Save Form Submissions` setting since most users don't turn that off.

If a user wanted to disable saving the forms they can do it site wide like this:

`add_action( 'ninja_forms_save_submission', '__return_false' );`

They can also do it on a per form basis by hooking into the same filter and using the `$ninja_forms_processing` variable that I pass along with the hook.