(Temporary) Disable v2 opens

[TheBlueMatt]

We've added a lot of code to enable inbound v2 (dual-funded, but without the ability to add our own funding) channels. There's a lot of risk in that much new code, so we should really have an option in the UserConfig to disable them at runtime.

[TheBlueMatt]

We instead decided to just yank v2 opens instead.