This is a follow-up for D144. It converts the wallet to use
`ContextualCheckTransactionForCurrentBlock()` instead of the
`CheckFinalTx()` wrapper and removes the unused finaltx.cpp/h files.
This is a preamble to backporting [[ https://github.com/bitcoin/bitcoin/pull/15288 | PR15288 ]] which will remove the direct call to `CheckFinalTx` in the wallet ([[ https://github.com/bitcoin/bitcoin/pull/15288/commits/80f52a2267f44a9cae4440615df3ff989be1579c| this commit ]]).
Having both `CheckFinalTx` and `ContextualCheckTransactionForCurrentBlock` is error prone when doing the node/wallet separation backports. This cleanup make it easier to backport and review.
Note that the above commit will allow to remove some of the boilerplate introduced in this diff.