Remove unneeded `Wallet` functions

[notmandatory]

As we were reviewing which Wallet functions to expose in the language bindings we found some functions that likely aren't needed by Wallet users. I'll use this issue to list which ones I don't think are needed or can be called directly on LocalChain. Comment below if you have any other suggestions.

I'd like to remove any unneeded ones now before we have a final 1.0 API.

[ValuedMammal]

I had another look at the API docs. Originally I thought get_psbt_input shouldn't be public but I don't really see a problem with it. We could possibly try to streamline the "stage" API - right now we have staged, take_staged, and staged_mut all with slightly different semantics.

Some other ideas for better organization are to reintroduce a unit test module for testing non public wallet functions, and consider making the common test utils available through a public test_util module.

[notmandatory]

fixed by #1658