Remove toggle-mode specific request observers

There might be a fair amount of performance overhead wrapped up in the multiple request observers in Torbutton. Many of these exist only because of hacks needed to safely support toggling and plugin-blocking. We can probably improve performance quite a bit by removing them.

This also makes work like #5709 (moved)/#1506 (closed) simpler, as android is rather sensitive to observers, and the multiprocess "Electrolysis" model makes some of them unreliable anyways.

See also #5965 (closed).