Move BrowserTestBase::$originalContainer to FunctionalTestSetupTrait

Actually this just looks like dead code? The property is only read, never written.

Sorry can the title/summary be tweaked? Mentions doing a move but seems just removing stuff?

If you are another contributor eager to jump in, please allow the previous poster(s) at least 48 hours to respond to feedback first, so they have the opportunity to finish what they started!

Meant to say you could self RTBC but thanks!

Committed/pushed to 11.x and cherry-picked to 11.2.x, thanks!

catch → closed merge request !12058