Fix rebase mistake in extension.json
This is a rebase mistake introduced with I7461ffd. Since If83420a this
module is not defined in extension.json any more, but in the
ResourceLoaderRegisterModules hook in PHP. This is done to be able to
dynamically change the contents of the module, depending on the
availability of anotehr extension.
I'm not exactly sure what happens when a module is defined twice. I
tried locally and it seems to work, with no visible error message.