Remove nursery/tmp_plan

[notmandatory]

The planning module has been added to rust-miniscript so this crate is no longer needed.

https://docs.rs/miniscript/latest/miniscript/plan/index.html

[notmandatory]

❓ Should the whole nursery folder be removed since in-development work can be done in a fork/feature_branch ?

[LLFourn]

Sure just get the examples to work without it?

[ValuedMammal]

I'm working on this