Remove node_add_body_field from NodeTypeForm

Alright, started an MR. Let's see what breaks.

Discussed this briefly with @Gábor Hojtsy in slack and he approved this change from a product manager perspective. It changes what happens with the form, but since the field is already deletable it doesn't affect other expectations outside the UI.

OK, so all PHP tests are now passing but Nightwatch tests are failing because they need to manually add the body field after creating a new content type. I have no Nightwatch experience.

Tried clicking the button in a different way etc. but did not really get anywhere.