This is a bug discovered while writing additional jtreg test in JDK-8224130.

add-modules is listed as an option that can be used as a key in an add-launcher properties file, but --add-modules is an option for creating the runtime image. There is only one runtime image used for the main app and all of the secondary apps created by --add-launcher option(s), so any option in an add-launcher properties file cannot have any impact on it.

If you want to (for example) create a non-modular main app and a modular secondary app you must either:
1.) put the modules used in the secondary app in modular jars and add them to --input dir.
2.) add the --module-path and --add-module options to the command line so that the runtime is built with the modules you need in the secondary launcher.

So we need to:
1.) remove add-modules from the list of supported keys in the help text of --add-launcher.
2.) remove code related to add-modules from AddLauncherArguments.