extensionUtils: Remove ShellJS library

extensionUtils: Remove ShellJS library

You can define a new importer object by importing a subdirectory in GJS.
This is undocumented, but it is likely to at least hold until the whole
thing moves to ES6 modules, after which we'll be able to do this purely
in JS with Reflect.Loader.

Since this was the only thing the ShellJS library did, we can remove it
altogether.

This allows us to discontinue use of the gjs-internals-1.0 embedder API.

https://bugzilla.gnome.org/show_bug.cgi?id=772386