libubox: add methods to delete elements from array and objects

[pprindeville]

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.