Temporarily disable WebRTC and the default browser agent on Windows

WebRTC support for mingw on 128 is broken because it needs some WinRT headers we're missing:

• windows.graphics.directX.direct3d11.interop.h
• wrl/event.h

and we need to update to a very recent version of Mingw (which also needs patches, see tor-browser#42758 (moved)).

The default browser agent was completely reworked between 115 and 128, so I haven't really updated the patch as needed during the rebase, but the default browser is kinda broken anyway, so it makes sense to come back to it when we are more ready.

For now, it makes more sense to disable these feature to get otherwise working builds for testing purposes.