Remove unused / noop offscreen rendering

At one point I think this might have helped make pages treated as
"visible" when added to the window, but it doesn't seem to have any
effect.

src/node/StreamWindow.js

@@ -69,9 +69,6 @@ export default class StreamWindow extends EventEmitter {
 
     const offscreenWin = new BrowserWindow({
       show: false,
-      webPreferences: {
-        offscreen: true,
-      },
     })
     this.offscreenWin = offscreenWin