Currently, Jetpack uses two sandboxes for each content script - one for the
content script itself, and another for SDK machinery. And we end up getting Xrays
between the sandboxes, even though they're same-origin (generally with an nsEP)
because Jetpack sandboxes use |wantXrays: true| to get same-origin Xrays.
Object Xrays cause all sorts of communication problems between the sandboxes.
Irakli says we don't actually need the separate sandbox anymore since we've
removed the proxy layer, so let's just get rid of it.
