libubox: add methods to delete elements from array and objects

Sometimes provisioning information will be provided in a JSON file at image build time that might turn out to be greater than what's detected at run-time. In those cases, it might be handy to be able to load a file, perform run-time validations that elements are or aren't present, and if the later delete them from objects and arrays before either iterating over that configuration, or saving it back out for later use.

To this end, please add methods like json_delete_nth which takes an integer (ordinal) n and deletes the n'th member of an array container, or else json_delete_key which deletes the member whose key matches key from an object container.

Start by locating the existing libubox JSON array and object APIs and any tests covering them. Add the requested json_delete_nth and json_delete_key entry points, then verify that array members or object members with matching keys can be removed before iteration or saving the JSON.