When $wgResourceLoaderSources was introduced, it required two keys, one for the load.php location, and another for the api.php endpoint. However, RL only actually uses the load.php location. The api part is unused, except for usage in ForeignAPIGadgetRepo in Gadgets 2.0.
I think it's inappropriate to have 'apiScript' in the core config, especially if nearly nothing uses it. The API link should be moved to a Gadgets configuration setting.