Can we merge `Channel::tx_complete` and `Channel::funding_tx_constructed`?

[TheBlueMatt]

Looks like we can? I think we maybe didn't because at the time those were written we didn't have the Channel-can-switch-state-internally stuff done, but now that we do we can probably merge them? cc @dunxen

[dunxen]

Yes, that's probably possible now. I'll leave it for after #3423.